Chapter 2 AAPG

7/24/2019 Chapter 2 AAPG

1/21

Depositional Themes of MixedCarbonate-siliciclastics in the SouthFlorida Neogene: Application toAncient Deposits

D. F. McNeill

Comparative Sedimentology Laboratory, University of Miami, Miami, Florida, U.S.A.

K. J. Cunningham1

Comparative Sedimentology Laboratory, University of Miami, Miami, Florida, U.S.A.

L. A. Guertin

Pennsylvania State University, Media, Pennsylvania, U.S.A.

F. S. Anselmetti

Geological Institute, ETH, Zurich, Switzerland

ABSTRACT

Arecent drilling project to evaluate the Neogene stratigraphy of south Florida has

provided refined insight to the depositional controls and facies patterns of a

heterogeneous, mixed carbonate-siliciclastic system. Six key themes have emerged

that may have implication for reservoir development and facies architecture in similar

depositional systems. These modern depositional themes are compared to some an-

cient mixed system examples. Although mixed systems are complex and spatially

unique, similarities in the basic lithofacies deposition and their associated physical

properties can aid in prediction of reservoir distribution and in refinement of geologic

models in ancient mixed systems. The deposition-related themes recognized in thisstudy of the Florida Neogene include (1) Concept of Template Control on Both Carbonate

and Siliciclastic Deposition precursor topography controls depositional geometry and

location of subsequent depocenters for both carbonates and siliciclastics; (2) Distal

Transport of Coarse Clastics and Influence of Currents on Grain-size Segregation conditions

can exist for the long-distance transport (fluvial?) of extremely coarse siliciclastics (flat-

pebble quartz in this Neogene example) from the source area, and regional currents help

segregate grain-size populations and partition grain types; (3) Demise of the Carbonate

2 McNeill, D. F., K. J. Cunningham, L. A. Guertin, and F. S. Anselmetti, 2004,

Depositional themes of mixed carbonate-siliciclastics in the south FloridaNeogene: Application to ancient deposits,in Integration of outcrop and modernanalogs in reservoir modeling: AAPG Memoir 80, p. 2343.

23

1Current affiliation: U.S. Geological Survey, Miami, Florida, U.S.A.


2/21

Platform/Ramp: Smothered by Siliciclastics? in this Neogene example, we recognize a

hiatus of several million years bounding the top of a carbonate ramp, which indicates

that demise of the ramp and subsequent input of siliciclastics are temporally distinct;

(4) The Mixing Transition: Abrupt Vertical and Lateral Facies Changes the lateral transition

of carbonate to siliciclastic strata highlights the potential for abrupt facies changes both

laterally and vertically. Interfingered carbonates and siliciclastics may form stratigraphic

traps based on lithologic differences and differential diagenesis and can result in alter-nating reservoir pay zones and nonreservoir intervals; (5) Cryptic Sequence Boundary in

Shallow-marine Siliciclasticsand Carbonates in cases where no distinct changein lithology

exists, it may be inherently difficult to recognize major disconformity based only on

lithologic changes. In settings dominated by admixing, sequence-boundary confirmation

may require the integration of biostratigraphic and chemostratigraphic markers with any

available textural indicators; and (6) Similarity in Acoustic Properties of Laterally Equivalent

Siliciclastics and Carbonates shallow burial and early diagenesis have produced an almost

identical acoustic signature for the two admixed sediment types. This acoustic similarity

may make it difficult to distinguish specific lithofacies on seismic profiles and sonic logs.

In ancient mixed-system deposits where only seismic data exist, problems in specific

lithofacies or geometric characterization may occur.

INTRODUCTION

To better understand the subsurface geology of the

southern Florida peninsula, the University of Miami

and Florida Geological Survey collaborated in the mid-

1990s on the collection of a series of continuously cored

borings in the Florida Keys and southern mainland of

Florida (Figure 1A). This drilling project was informally

termed the Florida Keys Drilling Project (FKDP). The

FKDP had three major objectives: (1) to better charac-terize the sediments and facies that comprise a rela-

tively young mixed carbonate-siliciclastic sedimentary

system; (2) to elucidate the complex and heteroge-

neous stratigraphic relations inherent in this system;

and (3) to provide a baseline case study on the physical

factors that influence the spatial distribution of mixedcarbonate-siliciclastic lithofacies and thus identify

characteristics of potential reservoir facies. These re-

sults form the basis for a case study applicable to an-

cient mixed systems. To that end, this paper distills

recently published results of the drilling and subsequent

analyses to compile six deposition-related themes.

The identification of these themes is based on the suc-cessful integration of data from cores, cuttings, geophys-ical logs, and marine-seismic profiles to provide new

stratigraphic, petrographic, chronologic, petrophysical,

and diagenetic information on these Neogene mixed-

system sediments and their distribution. The key pub-

lications from the FKDP include Warzeski et al. (1996),

Anselmetti et al. (1997), Cunningham et al. (1998), and

Guertin et al. (1999, 2000).

The six depositional themes have relevance to the

exploration and development of oil and gas reservoirs

in ancient mixed systems. The FKDP core-based stratig-

raphy, seismic data, and a regional compilation of ex-

isting cuttings and geophysical log data have provided

an age-constrained database for the establishment of

temporal and spatial relations between carbonate and

siliciclastic sediments. Data from this Neogene example

can be extrapolated to deep burial conditions, but the

physical changes (and range of variation) that result

from burial diagenesis should be considered. These

depositional themes are valuable in that the originaltextural lithofacies often directly influences the type

and degree of subsequent diagenesis. This combina-

tion of original texture and burial diagenesis is a major

control of porosity and permeability development in

ancient rocks. Fortunately, these facies relationships

and their influence on reservoir potential are com-monly predictable with a basic knowledge of sediment

source area, the mode of sediment input, mixing dy-

namics, and paleogeography. Results from this project

serve to provide a case study helpful to the under-

standing, conceptualization, and interpretation of an-

cient mixed-system lithofacies and their variability.

REGIONAL GEOLOGIC SETTINGOF THE MIXED SYSTEM

The late Neogene mixed carbonate-siliciclastic sys-

tem in south Floridaincludes two types of mixing (sensu

Budd and Harris, 1990). In vertical replacement mixing,

limestones and siliciclastics replace one another and

often result in alternating beds of each end-member

24 McNeill et al.


3/21

lithology. In lateral mixing, contemporaneous siliciclas-

tic and carbonate facies admix because of variability in

siliciclastic supply and/or lateral facies shifts. The keygeologic units in this study have been dated using a

combination of strontium-isotope stratigraphy (car-

bonates) and planktonic-foraminiferal biostratigraphy

(siliciclastics) (Figure 2) (Guertin, 1998; Guertin et al.,

1999). These refined ages constrainthe temporal nature

of mixed deposition with respect to major discontinu-

ities and proposed sea level changes (Haq et al., 1987,1988). Four formational units areincluded in the mixedsystem. Starting from the base, they include, first, the

lower to middle Miocene Arcadia Formation, a carbon-

ate ramp that is a composite sequence composed of four

high-frequency sequences (Cunningham et al., 1998).

The principal grains of the carbonate ramp are skeletal

fragments of mollusks, benthic foraminifera, red algae,

and echinoids, an assemblage of grain types consistent

with production in temperate water ( James, 1997). Al-

though predominantly carbonate, the Arcadia Forma-

tion can contain as much as several percent quartz sand

and phosphorite grains. The abundance of these non-

carbonate components increases from the Florida Keysnorthward across the peninsula. The top of the Arcadia

Formation is a regional unconformity that is replaced

by a thin, black phosphorite layer. The second for-

mational unit is an upper Miocene to upper Pliocene

mixed carbonate siliciclastic unit that is predominantly

siliciclastic in the northeast (Long Key Formation, as

much as 145 m thick) and carbonate in the southwest(Stock Island Formation,120 m thick). The newly de-fined Long Key Formation (Cunningham et al., 1998) is

a quartz-sand unit with varying amounts (


4/21

of fine-grained skeletal limestone with minor amountsof quartz sand that is laterally equivalent to the Long Key

Formation. The Stock Island Formation overlies the Ar-

cadia Formation and underlies the surficial Quaternary

limestones in the middle and lower Florida Keys (War-

zeski et al., 1996; Cunningham et al., 1998). Carbonates

of the Stock Island Formation mainly are very fine-

grained skeletal and planktonic-foraminiferal lime grain-

stone and lime packstone but contain some quartz-sand

grains (Cunningham et al., 1998). These carbonate sands

likely were transported by shallow currents flowing outof the Gulf of Mexico and across the progressively

shallowing shelf of the southern Florida peninsula.

Recent marine-seismic data suggest that beds of the Stock

Island Formation have prograded southward and east-

ward and downlapped onto the underlying Arcadia For-

mation (Warzeski et al., 1996). The third and fourth

formational units are the shallow subsurface deposits

and surface outcrops in the Florida Keys that consist of

Quaternary limestone of the Key Largo (reefal deposits)

26 McNeill et al.

FIGURE2. (A) Age-depth profiles constructed from strontium-isotope age ranges and planktonic-foraminiferal ageranges from Guertin (1998) and Guertin et al. (2000) for two cores from the Florida Keys. Note the extensive hiatusbetween the Arcadia Formation and the overlying Long Key and Stock Island Formations. (B) A comparison offormational ages of deposits in the south Florida mixed system with the proposed eustatic curve of Haq et al. (1987).The age information is from Guertin et al. (1999) and Cunningham et al. (2001a). The event ages of the sea levelcurve have been adjusted to the integrated timescale of Berggren et al. (1995) for direct comparison to the formation ages.


5/21

and Miami (oolitic sand) Formations. These well-studied,

shallow-water, tropical to subtropical deposits (Ginsburg,

1956; Stanley, 1966; Hoffmeister et al., 1967; Hoffmeister

and Multer, 1968; Perkins 1977; Shinn et al., 1989) rep-

resent the return to pure carbonate deposition and form

the cap to the south Florida mixed system.

The nature of carbonate-siliciclastic mixing has

evolved over time in south Florida because of changesin the amount of both detrital carbonate and siliciclas-

tic sediment that entered the basin. For example, in the

early and middle Miocene, only minor amounts of silici-

clastic sediment were transported to the (Arcadia For-

mation) ramp because of the lack of a proximal source

and a postulated physical separation by a strong current

flowing from west to east across the shelf margin. We

speculate that in the early and middle Miocene, the

southern peninsula was an open ramp setting that had

currents flowing across it from theGulf of Mexicoto the

east. Siliciclastic deposition decreased markedly from

north to south in the Arcadia Formation (Scott, 1988),

consistent with the hypothesis that some type of phys-

ical barrier, such as cross currents, blocked the south-

ward transport of voluminous quartz sand. Evidence

that currents physically blocked the input of quartz sand

to the Florida Keys area is the occurrence (Figure 1B, C)

of the overlying upper Miocene Peace River Formation

(Scott, 1988). The predominantly siliciclastic and phos-

phorite deposit is restricted to the Florida peninsula

in the early and middle Miocene. Quartz sand-rich de-

posits do eventually reach the Florida Keys by the early

Pliocene and perhaps even in the latest Miocene with

the deposition of Long Key Formation sediments (Guer-

tin et al., 1999; Cunningham et al., 2001a). The LongKey Formation likely is temporally equivalent to the

younger part of the Peace River Formation (Cunning-

ham et al., 2001a). The input mechanism of these

siliciclastic deposits has long been debated to be long-

shore transport or a fluvial-deltaic system centered in

the middle of the peninsula, or some combination of

the two. Several authors have suggested that the silici-

clastics were distributed down the peninsula as part of

a large prograding deltaic or river system, with depo-

sition in nearshore marine environments (Bishop, 1956;

Pirkleet al., 1964; Puri andVernon, 1964).More recently,a

hypothesis has been proffered that longshore processes

were the main mechanism for southward transport ofsiliciclastics (Alt, 1974; Kane, 1984). For the eventual

transport and deposition in south Florida, Ginsburg et al.(1989) suggested that a combination of fluvial, long-

shore, and shoreline transport processes were respon-

sible for forming an arcuate spitlike feature. This elon-

gate siliciclastic spit then provided a foundation shallow

enough for the deposition of Quaternary carbonates of

the Florida Keys. The latest clue in the mystery of the

siliciclastic source comes from the subsurface mapping

of cuttings, wire-line logs, and newly collected core

borings. Warzeski et al. (1996) proposed that a trend of

coarse-grained siliciclastics exists beneath the center of

the Florida peninsula, preserving what may have acted

as a sediment input pathway. Additional mapping and

coring helped Cunningham et al. (1998) refine this

siliciclastic corridor and identify several bifurcating

channel-like features that likely influenced local dis-

tribution of sand- and gravel-sized sediment into themarine basin. Cunningham et al. (2001b, 2003) used

high-resolution seismic and core borings to identify a

major fluvial deltaic depositional system about 200 km

north of the Florida Keys. This deltaic system may have

advanced as far south as the Florida Keys and beyond.

GEOLOGIC DATABASE

The evaluation of the south Florida mixed system is

based on analysis of five principal core holes. Four were

drilled by the Florida Geological Survey and continu-

ously cored, and one was drilled (partially cored) by the

Florida Keys Aqueduct Authority. The cores sampled a

cumulative thickness of 974 m. The cores were drilled

on Stock Island (near Key West), near Marathon and

Long Key(central Florida Keys), on KeyLargo (Carysfort

Marina), and on the mainland in the Everglades Na-

tional Park (Figure 1A).

In addition to the cores, data from approximately

200 wells (geophysical logs, cuttings shown in detail in

Cunningham et al., 1998) were used to determine lith-

ofacies distribution, formation thickness, and subsur-

face configurations. These well data were used to com-

pile cross sections and document the spatial nature ofthe mixed sediment types (Warzeski et al., 1996; Cun-

ningham et al., 1998) (Figure 3). The well locations of

the geologic database are shown in Figure 4A and B.

MIXED SYSTEMDEPOSITIONAL THEMES

Previous analysis of the cores and existing well data

(Warzeski et al., 1996; Melim, 1996; Anselmetti et al.,

1997; Cunningham et al., 1998; Guertin et al., 1999, 2000)

focused on specific aspects of the lithologic, stratigraphic,geochemical, and geophysical data collected as part of

the FKDP. The depositional-related themes formulatedand presented in this paper focus solely on the litho-

logic, stratigraphic, and petrophysical results of the proj-

ect. The aim of this thematic approach is to highlight

key results that may have bearing on similar aspects of

ancient mixed systems and especially those that host

hydrocarbon reservoirs. Several comparative (ancient)

examples of those from the south Florida example are

summarized after the following themes.

Depositional Themes of Mixed Carbonate-siliciclastics in the South Florida Neogene


6/21


7/21


FIGURE4. (A) Shaded structure contour map of top of the middle Miocene Arcadia Formation limestone. A channel-like depression running from just west of Lake Okeechobee to Florida Bay and beneath the Florida Keys was identifiedby Cunningham et al. (1998). This erosional or structural feature is hypothesized to have provided a pathway for theprogressive southward movement of siliciclastic sediments to the open basin now underlying Florida Bay and the FloridaKeys. Contour interval is in meters. (B) Isopach map of the cumulative thickness of core/cutting intervals containingquartz sand greater than 1 mm in diameter. The isopach contours are consistent with coarse sand and gravel movingthrough the channel-like depression in the underlying Arcadia Formation limestone. This limestone template hadsignificant influence on the transport pathway and eventual distribution of later siliciclastics. Contour interval is inmeters. (C) Core and log cross section of the southern peninsula illustrating the coarse-sand distribution. The coarsestsands are laterally restricted to a narrow corridor, consistent with a paleotopographic low. All three figures are modifiedfrom Cunningham et al. (1998) and used with permission of the Geological Society of America.


8/21

into a predominantly carbonate setting; the first point

likely is more appreciated by siliciclastic sedimentolo-

gists than by carbonate sedimentologists; and (2) re-

gional ocean currents help segregate and partition grain

sizes and grain types. The influence of currents is a pro-

cess that is difficult to identify in many ancient depositsbecause good three-dimensional data coverage is neededbut is often lacking. Thus, it often is an elusive process

in constructing depositional models and explaining lat-

eral changes in reservoirproperties. This Neogene exam-

ple illustrates how significant partitioning (segregation)

can be both in the depositional process and in the re-

sultant distribution of textural facies.

One of the most striking aspects of the Florida mixed

system is the presence of extremely coarse siliciclastics

beneath the modern Florida Keys (Figure 6). This intro-

duction of fluvial sand

and gravel is surprising

given that the prov-

ince has been carbon-

ate (and evaporite)

dominated since the

Jurassic. Available core

and cuttings data in-dicate that the spatial

limit of the coarsest

part of the siliciclastic

package has an elon-

gate geometry in aerial

view with a possible

channel-like configu-

ration (Warzeski et al.,

1996;Cunningham etal.,

1998) (Figure 4). This

elongate feature can be

traced from the Florida

Keys northward on the

Florida peninsula to

where it shallows to a

near-surface position

and fills an asymmet-

ric paleotopographic

low west of Lake Oke-

echobee. The ultimate

source of the siliciclas-

ticsis believed to be the

Appalachian Mountains

because it is the closest

source on the continent,but no specific prove-

nance data exist for the

siliciclastics. The silici-

clastics were intermit-

tently transported south-

ward along the Florida

peninsula (Figure 7),

mainly during the Miocene after the closure of the Su-

wannee channel across north Florida (McKinney, 1984).

Based on ourinformation andgeologic datato thenorth,

a fluvial-deltaic system has been proposed as the mech-

anism for the southward transport of siliciclastics more

than 1000 km down the Florida peninsula (Bishop, 1956;Pirkle et al., 1964; Puri and Vernon, 1964; Cunningham

et al., 2003). In south Florida, the coarse siliciclasticswere deposited in a small, shelf-edge marine basin that

currently underlies Florida Bay and the Florida Keys

(Cunningham et al., 1998).

On the Florida shelf, lithofacies partitioning be-

tween both carbonates and siliciclastics, as well as as-

sociated grain-size differences, are inherent because of

factors related to sediment supply, sediment depo-

center, physical conditions at the depocenter, and

30 McNeill et al.

FIGURE5. (A) Regional multichannel seismic line across the shelf, northern Florida Keys(off northern Key Largo). The relationship of the Miocene carbonate platform/ramp(Arcadia Formation) and the overlying prograded siliciclastic sediments is well illustrated.(B) Schematic core to seismic correlation in the middle Florida Keys showing the progradedsiliciclastics (Long Key Formation) overlying the Arcadia Formation. The top of the silici-clastic package provided a foundation for the subsequent initiation and deposition of Qua-ternary shallow-water carbonates. This siliciclastic template likely was a key factor in thechange from siliciclastic sedimentation back to carbonate sedimentation. The figure in (B)is modified from Warzeski et al. (1996) and is used with permission of the Society for Sedi-mentary Geology (SEPM).


9/21

hydrodynamic response of grain type/size to the phys-

ical setting. We propose that siliciclastics entering the

basin in the late Miocene and Pliocene were subjected

to strong currents flowing from the Gulf of Mexico

across the southern peninsula before combining withthe paleo-Florida current. A relatively strong currentlikely limited additional southward siliciclastic deposi-

tion and forced progradation of the finer-grained ma-

terial to the east and southeast (Figure 8). The coarsest

siliciclastics (>1 mm) remained along the western side

of the basin and formed a distinct corridor to the shelf

edge, now underlying the central part of the Florida

Keys. This segregation of carbonate and siliciclastic

material, as well as grain size, is related to the overriding

control of the shallow, eastward current flow. Litholog-

ically, these current-driven and prograding deposits

have distinct geographic ranges (Figure 8). Fine-grained,

bioclastic planktonic-foraminiferal grainstones and

packstones are found in the western Florida Keys,

whereas interfingering beds of quartz sand and forami-

niferal grainstone/packstone occur in the west-central

Florida Keys region. Siliciclastic coarse sand and gravel

underlie the central Keys, with a mixture of mostly finesand and mud-sized siliciclastics with planktonic fo-

raminifera and fine bioclastic debris toward the north-

eastern Florida Keys (Figure 8).


FIGURE6. (A) Coarse quartz-sand and gravel from a pieceof core from the Florida Keys Aqueduct Authority well atMarathon, in the middle Florida Keys. Quartz pebbles asmuch as 4 cm in diameter have been reported from someof the early core borings in the middle Keys (Vaughan,1910). (B) Flat-pebble quartz contained in parts of thecorridor of coarse siliciclastics. This specific sample is

from a sand quarry immediately west of Lake Okeechobee(Ortona, Florida). The transport of large, gravel-size quartzpebbles from the Appalachian Mountains to the FloridaKeys (1000 km) likely occurred through a fluvial-deltaicsystem that prograded down the Florida peninsula.

FIGURE7. Miocene paleogeographic reconstruction ofthe carbonate and siliciclastic environments based onMcKinney (1984). Siliciclastic transport most likely was

through fluvial-deltaic processes southward on the Floridapeninsula (approximated in figure by white arrows). Distaltransport of the siliciclastics down the Florida peninsulaprovided thesource for thecoarse sand andgravel beneaththe Florida Keys. Progressive southward movement of thesiliciclastics started in the late Oligocene (north Florida)and continued through the late Mioceneearly Pliocene,when they first arrived at the southern platform margin.Siliciclastic input to the depocenter beneath the FloridaKeys continued through the Pliocene and ended near thePliocene Pleistocene boundary (1.9 Ma) (Guertin et al.,1999).


10/21

As with both pure carbonate and siliciclastic basins,

the Florida mixed system was impacted by both re-gional (basin-scale) and smaller scale (local) processes.At the regional scale, the distal transport of very coarse

siliciclastic to a carbonate setting generated the mixing

of carbonate and siliciclastics. Locally, processes such as

strong currents generally separated the two sediment

end members and acted to partition the sediment type

and the particle size at the depocenter. As is common in

pure siliciclastic settings, this hydraulic partitioning

serves to provide distinct lateral changes in grain size

that could ultimately produce significant differences in

reservoir potential. Fortunate-

ly, however, this facies com-

partmentalization of sedi-

ment type and grain size

should occur in a predictable

distribution based on the un-

derlying templateand knowl-

edge of basin paleocurrents.

Demise of theCarbonatePlatform/Ramp:Smothered bySiliciclastics?

An integrated chrono-

stratigraphy (biostratigraphy,

strontium isotopes, magne-

tostratigraphy) for the car-

bonates and siliciclastics re-

covered by the FKDP has

provided refined ages of the

major lithologic units and

an estimate of hiatal dura-

tion at formation-bounding

surfaces (Figure 2A) (Guertin

et al., 2000). The age data in-

dicate that an approximately

8-m.y. hiatus in the Long

Key core occurs between the

carbonates of the middle

Miocene Arcadia Formationand the overlying siliciclas-

tics of the latest Miocene

and Pliocene Long Key For-

mation (Figure 9). A long-

standing dogma asserts that

carbonate platform demise

often is attributed to the

overwhelming influx of sili-

ciclastics (e.g., Meyer, 1989;

Smosna and Patchen, 1991).

This dogma (for which rela-

tively little evidence is found) suggests that carbonate

production essentially is shut down or greatly reduced toallow burial or drowning, respectively. The abrupt suc-

cession from a carbonate-ramp facies to an overlyingsand- and silt-rich siliciclastic (outer shelf) unit may be

(mistakenly) explained by this burial-drowned mecha-

nism. An interpretation of smothering especially is like-

ly if no age information exists and the lithologic in-

dicators of subaerial exposure (calcretes, soil breccias)

were not developed or preserved. Here, the regional

indication of erosion and/or flooding likely was respon-

sible for cessation of carbonate deposition. The upper

32 McNeill et al.

FIGURE8. Spatial distribution of Pliocene fine-grained carbonates (Stock IslandFormation) and siliciclastics (Long Key Formation). The siliciclastics have two sub-facies based on grain size: a mostly medium to coarse sand and gravel in a channel-likepattern and a finer-grained, mostly fine to medium, sand and mud, with some skel-etal carbonate that was deposited outside the coarse-sand corridor. The black arrowsin the figure are indicative of the transport direction for each of the main sedi-ment facies. White arrows indicate the direction of siliciclastic progradation basedon seismic data of Warzeski et al. (1996) and other unpublished data (numbered linesin the figure). The numbered lines show the location of unpublished seismic profiles.

The fine-grained carbonates likely were transported along the west Florida shelf andmixed with the siliciclastics along their western boundary. The finer-grained silici-clastics were transported to the south and were then influenced by eastward-flowingcurrents on the shelf. Seismic data (numbered lines) indicate eastward progradationof these sediments. The coarser-grained siliciclastics appear largely to have followed apaleotopographic depression southward down the peninsula. Their spatial distribu-tion indicates a bifurcation before moving beneath the area of the present FloridaKeys. Offshore seismic profiles suggest that the siliciclastics moved across the Mio-cene shelf (now exposed at the seafloor) and were deposited in deeper water in the Straitsof Florida.


11/21

boundary of the Arcadia is replaced by a thin layer of

black phosphorite (Cunningham et al., 1998), likely

deposited during intervals when the ramp was flooded

or was exposed to upwelling currents. In conjunction

with reduced sediment production caused by increased

ecologic stress, the Arcadia platform-ramp probably

was swept clean by intensified bottom currents asso-

ciated with late Miocene sea level changes. Of specialimportance in this case was the strong current originat-

ing in the Gulf of Mexico that flowed across the shelf

and joined with the northward-flowing paleo-Gulf

Stream current (Figure 7). In addition, stronger, more

erosive currents during lower sea level likely result

from several factors, as summarized by Brooks and

Holmes (1989): (1) the increase in current velocity re-

lated to the increased temperature gradient between

the equator and the poles (Brunner, 1983, 1986); (2) the

decreased cross-sectional area of the Straits of Florida

(Brunner, 1983); and (3) a downward shift in the po-

sition of the strengthened current and new contact

with previously deposited sediment.

For ancient mixed systems, the relevant point is

that the abrupt, distinctive change from carbonate to

siliciclastic sedimentation may not be attributable solely

to the influx of siliciclastics (excluding lowstand burial).

In some cases, a carbonate-to-siliciclastic contact rep-resents a hiatus of considerable duration, and the dis-

tinct lithofacies change is not necessarily one of cause

and effect. An incorrect interpretation of siliciclastic

smothering may have important ramifications on the

development of a regional geologic model. In fact, some

well-dated examples of the coexistence of carbonate

facies and voluminous siliciclastic input recently have

been recognized (Choi and Holmes, 1982; Friedman,

1988; Roberts and Murray, 1988; Santisteban and Taber-

ner, 1988; Esker et al., 1998; McNeill et al., 2000). Con-

versely, documented examples of regional carbonate

platform demise by siliciclastic smothering are rela-

tively rare, even for modern tropical settings where high-

resolution dating can establish a conformable transition.

Until the (conformable) timing of carbonate to silici-

clastic transition is established, a burial-demise effect is

not the sole interpretation of these distinct boundaries.

In fact, smothering might be the exception rather than

the rule. Indeed, other reasons such as sea level rise (the

drowning unconformity of Schlager and Camber, 1986),

environmental deterioration (Hallock and Schlager,

1986; Erlich et al., 1993) and/or the influence of local

ocean currents present viable alternative hypotheses

(and evidence) to that of conformable siliciclastic buri-

al and carbonate demise.

The Mixing Transition: Abrupt Verticaland Lateral Facies Changes

The lateral transition of carbonate to siliciclastics

in the south Florida example highlights the potential

for abrupt lateral and vertical facies changes in mixed

environments. The setting for carbonate-siliciclastic

mixing often is one with two distinct sediment sources,

and in most cases, several different controls on the ac-

cumulation of that sediment. The examination of cores

and cuttings from the central Florida Keys shows in-

terfingering between two distinct lithologies: the fine-grained skeletal fragment and foraminiferal grainstone-

packstone of the Stock Island Formation and the quartzsandstone of the Long Key Formation (Figure 1C). Al-

though lithofacies of these two formations interfinger,

considerable admixing still occurred during their depo-

sition. At the interfingering transition, variable amounts

of quartz sand are included in the carbonates of the Stock

Island Formation, and conversely, skeletal carbonate is

contained in the predominantly siliciclastic lithology of

the Long Key Formation. We interpret the fine-grained


FIGURE9. Contact between the Arcadia Formation car-bonates and the Long Key Formation siliciclastics (whitearrow) in the Long Key core (630 ft; 192 m). The abruptcontact could be interpreted as carbonate platform demisecaused by the sudden influx of siliciclastics. Age dates forthe two formations, however, suggest that a hiatus of ap-proximately 8 m.y. occurs between the two units (Guertin,1998; Guertin et al., 2000).


12/21

bioclastic and planktonic-foraminiferal carbonates to

be a shelf current deposit that was admixed with some

quartz sand (Figure 3). These fine-grained carbonateswere transported by a west-to-east flow of the current

across the west Florida shelf. The carbonates accumu-

lated in a fairway along the southwestern edge of the

Florida platform, whereas in the east, they were mixed

with the siliciclastics introduced from the north. Pre-

sumably, the eastward-flowing current precluded large

amounts of siliciclastics from being transported to thesouth and west where the fine-grained skeletal sedi-ment accumulated. Likewise, siliciclastic deposition was

restricted to the central and eastern part of the Florida

platform by eastward-flowing currents. These eastward

currents incorporated some of the fine-grained skeletal

carbonate with the siliciclastics. The actual zone of

lateral mixing is constrained to an area with a width of

about 25 km (from W-5152 to W-1299 in Figure 1C),

now underlying the central-southern Florida Keys

(Figure 10). Core data indicate that this zone of lateral

mixing migrated back and forth

repetitively through time, while

maintaining overall its general

position (Cunningham et al., 1998)

(Figure 11). To produce this inter-

fingered configuration, we specu-

late that a combination of sea lev-

el and current-velocity changescontrolled the amount of silici-

clastic sediment transported to

the mixing transition.

The nature of this mixing

boundary is applicable to ancient

mixed systems in two ways. First,

in a hydrocarbon-bearing setting,

lithologic differences produced by

a shifting carbonate-siliciclastic

facies boundary may form subtle

stratigraphic traps. For example, if

one of the lithofacies in a section

of interfingered beds is a more

favorable reservoir rock (e.g., high-

er porosity and permeability) and

the other is relatively tight, the

lateral extent of these more por-

ous beds could produce subsurface

reservoir zones in the transition

zone (Figure 11). In a well, the res-

ervoir units would appear to be

stacked, separated by the nonres-

ervoir lithologies. The control on

the positioning of these lateral

facies shifts is likely externallycontrolled and may be correlative

with other indicators of current

intensification and/or sea level

change recorded in the basin. Second, this mixing tran-

sition, however, may be difficult to image acoustically

if siliciclastics and carbonates are uniformly mixed and

are of similar acoustic character. Unless appropriate

impedance contrast is developed by diagenetic en-

hancement of one lithology relative to the other, the

mixing transition on seismic profiles may be spatially

unresolvable.

Cryptic Sequence Boundary in MixedSiliciclastics-carbonates

The mixed siliciclastic-carbonate sediments recov-

ered in cores from Long Key and Key Largo (Carysfort

Marina) (Figure 1A), although lithologically similar,

span an interval known to contain a regionally estab-

lished sequence boundary (equivalent to the Miocene

Pliocene TB3.3 3.4 boundary of Haq et al., 1987). This

sequence boundary also has been documented in the

34 McNeill et al.

FIGURE10. Paleogeography of the Long Key (siliciclastic) and Stock Island(carbonate) Formations. Throughout their deposition, the lateral boundary betweenthe two units has remained generally stationary within a 25-km band (see crosssection in Figure1C). This interfingering transition is manifest in thesubsurface byalternating beds of limestone and sandstone (both with a minor component of theother sediment type). These alternating lithologies apparently pinch out laterallyin the 25-km transition zones defined by our well control. Similar alternatinglithologies could form subtle stratigraphic traps givendifferent diagenetic changesand porosity evolution, for example, the Yates Formation in the Permian Basin(Borer and Harris, 1991a, b). Figure modified from Cunningham et al. (1998) andused with permission of the Geological Society of America.


13/21

same basin (but different platform) on western Great

Bahama Bank but in the pure carbonate lithofacies

(Eberli et al., 2001; McNeill et al., 2001). By definition,the most pronounced attributes of a sequence bound-

ary in siliciclastic sediments are seismic truncation, a

basinward shift in facies, and subaerial exposure in up-

dip locations (Van Wagoner et al., 1990). In shallow-

marine settings, a facies shift commonly is associated

with an abrupt grain-size increase and/or a distinct up-

ward change from limestone to sandstone. In the twoFlorida cores where no distinct lithologic change oc-curs, biostratigraphy (change in foraminiferal coiling

direction, change in benthic-foraminiferal fauna, posi-

tion of a prominent first appearance) has helped con-

strain this sequence boundary to within an interval of

several meters (Figure 12). The location of the bio-

stratigraphic change is coincident with an upward-

coarsening in quartz-sand grain size and a slight change

in color of the dry sediment (Figure 13). Confirmation

of the TB3.33.4 sequence boundary in this mixture of

siliciclastic-carbonate sediment

was uncertain using only textural

properties or lithology. Where di-

agnostic subaerial exposure features

are absent and no distinct lithologic

change occurs (as in this case), we

have found that the disconformity

can be reliably constrained withthe use of foraminiferal microfos-

sil data in conjunction with grain-

size changes across the boundary

(Figure 14).

The relatively shallow-water

nature of some mixed systems

makes recognition of the sequence

boundary inherently difficult in

cases where a distinct change in

lithology is absent or the section is

located in a downdip position that

precludes subaerial exposure. In

this Florida example, we have found

that biostratigraphic markers are

key indicators of hiatal discon-

formity, especially when used with

other sedimentological changes.

The mixed lithofacies immediately

above the disconformity contain

the first occurrence of an age- and

environment-diagnostic microfos-

sil relative to nearly identical litho-

facies below the disconformity.

This example illustrates that fea-

tures associated with a disconform-ity, especially where no lithologic

change occurs, can sometimes be

extremely subtle. These cryptic se-

quence boundaries, however, can be recognized with

the integration of the lithologic, biologic, and chemo-

stratigraphic data.

Similarity in Acoustic Properties ofLaterally Equivalent Siliciclasticsand Carbonates

Anselmetti et al. (1997) studied the sonic velocityof the MiocenePliocene prograding siliciclastics and

carbonates from the subsurface of the Florida Keys.They found that sonic velocity in these mixed-system

sediments is controlled by threefactors: (1) porosityand

primary pore type, (2)quartzcontent, and(3) dolomite-

limestone proportion. In this case, the combination of

depositional fabric and diagenetic alterations has pro-

duced almost identical impedance, yielding similar

p-wave velocity for the fine-grained carbonates, the

siliciclastics, and the admixtures (Figure 15). The mixed


FIGURE11. Lithology, log signature, and partial mineralogy record from theFlorida Keys Aqueduct Authoritycore OW-1 located in Marathon (seeFigure 1a forlocation). This well penetrated the mixing transition between the siliciclastics andcarbonates. The core lithology shows alternating beds of predominantly lime-stone and quartz sandstone intervals, both with a minor component of the othersediment type admixed. The interfingered lithofacies likely result from shifts inthe relative input of carbonate and siliciclastic sediment, perhaps related toeustatic sea level changes. LMC = Low-Mg Calcite.


14/21

carbonates and siliciclastic sediments of the Stock Is-

land and Long Key Formations both are characterized

by low seismic reflectivity or even seismic transpar-ency. The similarity of core-based sonic velocity be-

tween the carbonates, siliciclastics, and admixtures of

the two led to difficulty in identification of lithofacies

from sonic logs, synthetic seismograms, and offshore

marine seismic data (Warzeski et al., 1996; Anselmetti

et al., 1997). Consequently, for the Florida mixed sys-

tem, in offshore areas where no well data were avail-able, seismic data could not be used to distinguish be-tween the prograding quartz sand and the prograding

fine-grained carbonates.

The significance as applied to ancient mixed sys-

tems is that it may be difficult to map lateral changes

from seismic data between two time-equivalent mixed

units because of similar acoustic character. Recognition

might eventually depend on the type and amount of

burial diagenesis, assuming that this diagenesis will gen-

erate a contrast in sonic velocity. The significant point

here is that the combination of depositional character-

istics and diagenetic factors may, on occasion, produce

sonic velocity signatures that are remarkably similar.This acoustic similarity can make it very difficult to

discern predominantly carbonate deposits from silici-

clastic deposits, but conversely, it may also be a useful,

distinctive signature of the admixed lithofacies.

COMPARISON TO SOMEANCIENT EXAMPLES

The depositional themes compiled from the Neo-

gene mixed sediments of south Florida were formulated

for the purpose of highlighting common processes that

are fundamental to the interpretation of other mixed

carbonate-siliciclastic deposits. Each mixed depositional

setting has individual variations in the source, the vol-

ume, the transport mechanisms of siliciclastic sediment,

36 McNeill et al.

FIGURE12. A summary and comparison of key features found across a regionally established early Pliocene sequenceboundary (SB) (TB3.3 3.4 of Haq et al., 1987) in the Florida Bahamas region. The sequence boundary is lithologicallysubtle in the Florida mixed-system sediments, and biostratigraphic data was used to confirm and constrain its position.This mixed quartz sand (85%) and low-magnesium calcite (LMC) (15%) mineralogy, both above and below the

boundary, poorly records or preserves features indicative of a discontinuity. The carbonate end member is considerablybetter at recording diagenetic changes associated with erosion, nondeposition, and subaerial exposure. Florida data arefrom Guertin (1998). Core Clino and core Unda data are from Kenter et al. (2001).


15/21

and variations in the carbonate platform or ramp mor-

phology. These individual variations are further impact-

ed by the regional physical influences that result from

the change in sea level, intensity of ocean currents, and

the prevailing climate regime. To apply some of the dep-

ositional themes derived from this Neogene example,

several select ancient examples are cited, in whichsimilarcontrols or processes may have impacted mixed-systemdeposition.

The concept of template control, where precursor

topography influences subsequent deposition, is not

new. That reef- or karst-generated paleotopographic

highs are sites for the reoccupation of reefs during sub-

sequent highstands is a widely accepted concept. The

idea of paleotopography in limestone surfaces control-

ling siliciclastic sedimentation and the idea of silici-

clastics providing the foundation for carbonates also

are not uncommon but are less widely documented

(some Cenozoic examples: Gvirtzman and Buchbinder,

1978; Choi and Holmes, 1982; Santisteban and Taber-

ner, 1988; Friedman, 1988). For a carbonate template,

Handford and Loucks (1993) noted the importance of

karst processes in the development of paleotopogra-

phy, as conceptualized in their sequence model for a

humid carbonate-siliciclastic rimmed shelf. In addition,of key importance is the influence of structural modifica-

tion on carbonate platforms and the generation of relief

as a controlling template. Several key examples can be

found in the literature regarding the importance of a

carbonate template in siliciclastic sedimentation. Cu-

zella et al. (1991) have documented the importance of

Mississippian carbonates of central Kansas as a control-

ling template. They have shown that basal Pennsylva-

nian conglomerates and fluvial sands (siliciclastics) are

associated distinctly with valleys developed in the un-

derlying carbonates. Thecoarser conglomeritic bedsprob-

ably were deposited in braided streams that were later

reworked by coastal processes. Simo (1989) illustrated a

similar example in the Upper Cretaceous Tremp Basin

(Aren sequence), where the Aren subsequence 5-B lower

boundary is characterized by karst topography and

incised valleys cut into the karst surface. The incised

valley depressions in the underlying limestone contain

quartz-pebble conglomerates, a clear example of paleo-

topography-controlled sedimentation. Likewise, the con-

tribution of siliciclastics as the foundation for carbonate

deposition is nicely demonstrated by an example from

the FrasnianFamennian reef complex of the Canning

Basin, Western Australia (Southgate et al., 1993). This

analysis of seismic and well data suggests that majorlowstand siliciclastic deposits on the shelf margin pro-

vided a foundation for the overlying transgressive and

highstand reefal and platform carbonates. Cenozoic

examples of the importance of siliciclastics as a reefal

foundation are more well known, as cited above. This

siliciclastic-to-carbonate succession is a key component

of the template concept and likely has been operative

over much of the Phanerozoic, especially important

during times when reef framebuilders were dominant.

The distal transport of coarse siliciclastics (gravel

size) into predominantly carbonate basins is a process

that likely is underappreciated and underrecognized by

carbonate sedimentologists. Of course, this excludes set-tings where a nearby source of coarse siliciclastic sed-

iment is admixed with shallow-water carbonates, forexample, the Devonian reefs of the Canning Basin

(Playford et al., 1989; Holmes and Christie-Blick, 1993).

Conceptually, Handford and Loucks (1993, p. 19) have

described this process. They state that under lowstand

conditions, fluvial-deltaic processes . . .can spread

siliciclastic sediments far and wide across subaerially

exposed platforms during lowstands. The Neogene

example from Florida is one such example of this type


FIGURE13. Core photograph of the early Pliocene TB3.33.4 sequence boundary (SB) in the Carysfort Marina core. Aslightchange in grain size and color are noticeable across theboundary. Planktonic- and benthic-foraminiferal data haveaided in constraining this sequence boundary, a boundary

where the disconformity is not especially distinct basedsolely on lithologic characteristics. Note, however, thatthere are gaps in core recovery and that some other in-dicative characteristics of the boundary may not havebeen recovered.


16/21

of deposition, albeit with not only sand-sized material,

as in most cases, but also with flat-pebble-sized material

as much as 4 cm in diameter. An interesting example of

distal transport recently was reported by Brenner et al.

(2001), who documented the long-distance (>500 km)transport of pebbles and small cobbles via fluvial pro-

cesses. They describe middle Cretaceous coarse siliciclas-

tics that were deposited in incised valleys cut in Paleozoic

strata of Iowa and eastern Nebraska, along the eastern

margin of the Western Interior Seaway. Brenner et al.

(2001) have speculated that long-distance transport was

related to seasonal monsoon climatic conditions thatcharacterized the region during the middle Cretaceous.A second example of pebble and cobble transport (of at

least 100 km) is the example described from the Albian

mixed systems of northern Spain by Garca-Mondejar

and Fernandez-Mendiola (1993). They describe quartz-

ite and sandstone pebbles that followed a paleokarst

limestone surface and likely were transported into the

predominantly carbonate setting by strong fluvial cur-

rents during subaerial exposure. Garca-Mondejar and

Fernandez-Mendiola (1993) suggest that the fines were

selectively removed from the siliciclastics during the

subsequent transgression, a partitioning scenario very

similar to that proposed in our south Florida example.

The distal transport of fine and medium sand-sized

material and both horizontal and vertical mixing withcarbonate sediments is a relatively common process,

and numerous examples (i.e., the Permian Basin) exist.

Although the input of gravel-sized material is less com-

monly reported, we propose that it can be a significant

component of mixed carbonate-siliciclastic deposition-

al systems when an adequate source exists.

The demise of carbonate platforms by the influx ofsiliciclastics (lowstand burial excluded) still is a widelydebated topic. From a sequence-stratigraphic viewpoint,

Schlager (1989, p. 17) argued that For the final geom-

etry, it makes little difference whether a platform was

killed by rapid submergence and later buried by silici-

clastics, or whether burial by the siliciclastics caused

the demise. From a geologic standpoint, however, the

timing of siliciclastic input relative to platform demise

is important for both regional depositional models

as well as reservoir models and seal prediction. An

38 McNeill et al.

FIGURE14. Lithologic and paleontologic characteristics of the mixed siliciclastic-carbonate sediments in the Long KeyFormation. Changes in planktonic foraminifera coiling direction, benthic foraminifera depth indicators, age-diagnosticfirst occurrence (FO), and slight grain-size differences have contributed to the constraint of the sequence boundaryposition. The outer shelf (o.s.) and inner shelf (i.s.) designations are from benthic foraminifera habitat ranges. Thespatial difference and transitions in each sequence from outer shelf to inner shelf probably reflect the point-sourceinput of the siliciclastics. The figure is reproduced from Guertin et al. (1999) and used with permission of the Societyfor Sedimentary Geology (SEPM). Intervals I, II, and III represent the key depositional intervals as determined byGuertin et al. (1999).


17/21

interpretation that commonly is proposed for a sharp

boundary in which siliciclastics overlie carbonate is oneof decreased or terminated carbonate production caused

by smothering or burial. The Florida Neogene example

adds to the growing evidence that not all deepening-

upward, carbonate-to-siliciclastic sequences are a result

of a burial smothering mechanism. The approximately

8-m.y. hiatus between the final carbonate deposition

and the first siliciclastic accumulation clearly points tosome other cause of platform demise. An excellentexample of a carbonate platform subjected to environ-

mental stress prior to siliciclastic burial and the occur-

rence of a considerable hiatus between the carbonate

and siliciclastic deposition was described by Erlich et al.

(1990, 1993) for the Upper JurassicLower Cretaceous

carbonate platform of the Baltimore Canyon area.

The horizontal mixing and stratigraphic interfin-

gering of carbonates and siliciclastics described in the

Florida example is similar to many ancient platform mar-

gin and shelf deposits (e.g., Cam-

brian of central Texas, King and

Chafetz, 1983; Upper Permian of

the Permian Basin, Mazzullo et al.,

1991; Upper Cretaceous of Angola,

Lomando and Walker, 1991). One

classic example is the Yates Forma-

tion in the Central Basin Platformin the Permian Basin (Borer and

Harris, 1989, 1991a, b; Mutti and

Simo, 1993), and others exist in

the same basin (e.g. the Queen

and Grayburg Formations). In the

Yates Formation example of Borer

and Harris (1991b), a series of cores

along a dip-oriented transect docu-

ments the lateral mixing of silici-

clastics andcarbonates at a middle-

shelf location behind the carbon-

ate-rich shelf margin. The main

zone of mixed sediment on the

middle shelf was about 25 30 km

wide. The cyclic nature of the silici-

clastic and carbonate beds on the

middle shelf, in conjunction with

biostratigraphic age control, led to

the interpretation that low-ampli-

tude sea level changes caused by

orbital forcing were the main con-

trol on sediment type. If interfin-

gered carbonates and siliciclastics

of the Yates Formation were a

result of high-frequency sea levelchanges, these alternating lithol-

ogies appear also to have con-

trolledthe eventual reservoir prop-

erties(Borer andHarris, 1991b). In

the Yates, siliciclastic sand(lowstand deposit) formsthe

main reservoir units and is interbedded with nonreser-

voir carbonates (highstand deposit). In the lowstand

siliciclastics, spatial differences exist in reservoir quality

because of local controls such as the steeper depositional

slope, locally faster subsidence, and the amount of win-

nowing related to current energy. We might expect some

of the same controls to have produced the interfingered

carbonate-siliciclastic facies of the Florida example. Even-tual reservoir properties likely will be determined by

similar depositional influences: high-frequency sea lev-el changes, sediment grain-size partitioning by currents,

and progressive differences in diagenesis and cemen-

tation between adjacent beds. These influences might

ultimately produce stacked reservoir and nonreservoir

intervals, as found in the Yates Formation example.

The acoustic characteristics of mixed carbonate-

siliciclastic sediments were found to be nearly identical

(low reflectivity, nearly transparent acoustically) in


FIGURE15. Sonic-velocity logs and discrete-sample velocity data from the StockIsland and Long Key Formations. The carbonates of the Stock Island Formationand the siliciclastics of the Long Key Formation have remarkably similar acousticcharacteristics. This similarity makes the seismic determination of lithofaciesextremely difficult using seismic data alone. Several ancient examples discussed

in the text show that siliciclastic and carbonate admixtures sometimes retain thisacoustic similarity through burial. The figure is from Anselmetti et al. (1997) andused with permission of Elsevier Publishing.


18/21

the Florida example. Thus, it was difficult to image the

anatomy of horizontally mixed and interfingered

facies using seismic data alone. In most cases, however,

we would expect burial diagenesis to produce some con-

trast in acoustic signature of the interbedded limestones

and sandstones. Where the two sediment types are ad-

mixed on the shelf margin or ramp setting, the acous-

tic signature also can be nearly transparent. Several keyexamples exist for shelf-margin deposits similar to those

in the Florida Neogene. Southgate et al. (1993) have

documented a Late Devonian Mississippian mixed

carbonate-siliciclastic ramp system in which highstand

deposits lack internal reflection. These acoustically trans-

parent platform deposits apparently make recognition

of seismic-based lithofacies or geometries difficult in

the highstand deposits. Fortunately, cuttings and cores

were available to document the admixed and inter-

bedded nature of the sandstone, mixed sand dolostone,

and dolostone lithofacies on the inner ramp (Southgate

et al., 1993). In another interesting example, Meyer

(1989) discusses the influence of siliciclastics on the

Mesozoic platform of the Baltimore Canyon trough. In

the shelf-margin system (KimmeridgianBerriasian), he

describes admixing and interbedding of quartzose sand

and silt with shallow-water carbonate grainstone and

framestone textures. This shelf-margin mixed system

has velocity logs that correlate to a seismically quiet zone

with poor reflection characteristics. Acoustic reflectiv-

ity improvessubstantiallyoutward fromthe shelf-margin

system into both the adjacent slope and shallow-shelf

deposits where admixed carbonates and siliciclastics

are less prevalent. In cases for which only seismic data

exist, the low reflectivity or transparent character mayeven be a useful tool in identifying and mapping mixed

deposits.

SUMMARY OFCOMPARATIVE THEMES

Six depositional themes for a late Neogene mixed

carbonate-siliciclastic system have been identified to

help understand facies relations of ancient carbonate-

siliciclastic deposits. An improved understanding of

the factors that influence mixing is necessary to gen-

erate accurate regional geologic models and providesome predictability to these spatially and temporally

complex facies. Although each mixed system is uniquewith respect to facies geometry, spatial scale, and sedi-

ment input, once understood, the relationship between

carbonates and siliciclastics becomes considerably less

intimidating.

These fundamental themes can be applied at dif-

ferent scales. At the regional geology scale, and with

limited two-dimensional seismic data and only sparse

well and log data, the explorationist should be aware of

relatively abrupt, lateral lithofacies changes that can

occur in mixed carbonate-siliciclastics settings. That

these lithofacies changes may not always be distinguish-

able in seismic data is especially important. With pro-

gressive burial, the hope is that diagenesis will develop

sufficient petrophysical contrasts to distinguish the

main lithofacies in log and seismic data. Similarly, at

the regional scale, the geophysical log signature of mixedsiliciclastics-carbonates can be nonunique, making ac-

curate lithofacies interpretations difficult when cuttings

are unavailable. In a more positive slant, the predictive

capability of the petroleum geologist can be enhanced

significantly with conceptualization of a depositional

template(either erosional or structural).It has long been

realized that reefs beget reefs, but probably just as im-

portant is that limestone paleotopography can have

important control on siliciclastic distribution and fa-

cies, andconversely, that siliciclastics canprovide a foun-

dation for carbonate initiation and development where

none existed previously. Thus, where feasible, geologic

modeling should evaluate and try to incorporate the

effects of a paleotopographic template in the mapping

of reservoir facies.

At the more local and development scale, the long-

standing (and largely incorrect) dogma that siliciclas-

tics limit carbonate deposition has been challenged

with numerous recent examples of reef-siliciclastic co-

existence. As such, the nature of a vertical lithofacies

change from carbonate to siliciclastic needs to be cau-

tiously interpreted with respect to cause and effect in

regional geologic models. It should also be recognized

that coarse siliciclastics (gravel as well as sand-sized

sediment) can be transported long distances into pre-dominantly carbonate settings. More importantly, lo-

cal processes at the depocenter (in this case, the in-

teraction of fluvial deltaic and ocean-current processes)

canact to sort the transported material by grain size and

hydraulic equivalence. In the south Florida example,

fluvial and ocean-current processes concentrated and

segregated the predominantly coarse and fine silici-

clastic sediment. This hydraulic sorting and partition-

ing can form reservoir facies with compartmentalized

properties, especially porosity and permeability. The

south Florida example serves to illustrate the impor-

tance of evaluating local and regional ocean currents in

coastal basins as part of the overall basin analysis mod-el. These current-related processes may be just as im-

portant as eustatic sea level changes in sculpting thelocal sequence geometry. Last, the mixing transition

for laterally equivalent carbonates and siliciclastics may

be important in generating subtle stratigraphic traps as

burial progresses and different diagenetic facies evolve.

In this case, the mixing transition appears to be abrupt

at the regional scale, although in reality, the inter-

fingering of limestone and siliciclastic facies over a

40-km width would provide both the thickness and

40 McNeill et al.


19/21

spatial dimension for subtle stratigraphic traps. Simi-

larly, stacked reservoir and nonreservoir units may result

from thehorizontal and vertical mixing at the carbonate-

siliciclastic transition.

ACKNOWLEDGMENTS

This drilling project was made possible through

assistance of the Florida Geological Survey. Theirskilled

team provided exceptional recovery of cores at all the

drill sites. We appreciate the efforts of Tom Scott and

Ken Campbell, who managed the survey drilling pro-

gram. Robert Ginsburgs longstanding efforts and in-

terest in the siliciclastics beneath south Florida were the

impetus for the (long overdue) drilling. The project also

benefited greatly from the initial work of Robert War-

zeski. Financial support of the project came from the

donors of the American Chemical Society the Petro-

leum Research Fund and the industrial associates (Chev-

ronTexaco, ConocoPhillips, Total, ExxonMobil, Japan

National Oil, Encana, Shell, and Statoil ASA) of the

Comparative Sedimentology Laboratory, University of

Miami. The thoughtful, constructive reviews of Gene

Rankey, Sal Mazzullo, and Mitch Harris are greatly

appreciated.

REFERENCES CITED

Alt, D., 1974, Arid climate control of Miocene sedimenta-tion and origin of modern drainage, southeastern

United States,in R. Q. Oaks Jr. and J. R. Dunbar, eds.,Post-Miocene stratigraphy, central and southernAtlantic Coastal Plain: Logan, Utah, Utah State Uni-versity Press, p. 2129.

Anselmetti, F. S., G. A. von Salis, K. J. Cunningham, andG. P. Eberli, 1997, Controls and distribution of sonicvelocity in Neogene carbonates and siliciclastics fromthe subsurface of the Florida Keys: Implications forseismic reflectivity: Marine Geology, v. 144, p. 9 31.

Berggren, W. A., D. V. Kent, C. C. Swisher III, and M.-P.Aubry, 1995, A revised Cenozoic geochronology andchronostratigraphy, in W. A. Berggren, D. V. Kent,M.-P. Aubry, and J. Hardenbol, eds., Geochronology,time scales and global stratigraphic correlation: SEPM

Special Publication 54, p. 129212.Bishop, E. W., 1956, Geology and ground water resourcesof Highlands County, Florida: Florida GeologicalSurvey, Report of Investigation 15, 113 p.

Borer, J. M., andP. M. Harris, 1989, Depositionalfaciesandcycles in Yates Formation outcrops, Guadalupe Moun-tains, NewMexico, in P.M. HarrisandG. A.Grover, eds.,Subsurface and outcrop examination of the Capitanshelf margin, northern Delaware basin: SEPM CoreWorkshop 13, p. 305317.

Borer, J. M., and P. M. Harris, 1991a, Depositional faciesand model for mixed siliciclastics and carbonates of

the Yates Formation, Permian Basin, inA. J. Lomandoand P. M. Harris, eds., Mixed carbonate-siliciclasticsequences: SEPM Core Workshop v. 15, p. 1133.

Borer, J. M., and P. M. Harris, 1991b, Lithofacies andcyclicity of the Yates Formation, Permian Basin:Implications for reservoir heterogeneity: AAPG Bul-letin, v. 75, p. 726779.

Brenner, R. L., G. A. Ludvigson, B. J. Witzke, P. L. Phillips,T. S. White, D. F. Ufnar, and R. M. Joeckel, 2001,Transporting coarse-grained material long distancesover low gradients: Cretaceous conglomerates fillingincised valleys on the cratonic margin of the WesternInterior Foreland Basin (abs.): AAPG Annual MeetingProgram, v. 10, p. A25.

Brooks, G. R., and C. W. Holmes, 1989, Recent carbonateslope sediments and sedimentary processes borderinga non-rimmed platform: Southwest Florida conti-nental margin, in P. Crevello, J. L. Wilson, J. F. Sarg,and J. F. Read, eds., Controls on carbonate platformand basin development: SEPM Special Publication 44,p. 259272.

Brunner, C. A., 1983, Evidence for increased volumetransport of the Florida Current in the Pliocene andPleistocene: Marine Geology, v. 54, p. 223235.

Brunner, C. A., 1986, Deposition of a muddy sedimentdrift in the southern Straits of Florida during the lateQuaternary: Marine Geology, v. 69, p. 235249.

Budd, D. A., and P. M. Harris, eds., 1990, Carbonate-siliciclastic mixtures: SEPM Reprint Series No. 14, 272 p.

Choi, D. R., and C. W. Holmes, 1982, Foundations ofQuaternary reefs in south-central Belize lagoon,Central America: AAPG Bulletin, v. 66, p. 2663 2671.

Cunningham, K. J., D. F. McNeill, L. A. Guertin, P. F.Ciesielski, T. M. Scott, and L. de Verteuil, 1998, NewTertiary stratigraphy for the Florida Keys and south-

ern peninsula of Florida: Geological Society ofAmerica Bulletin, v. 110, p. 231258.Cunningham, K. J., D. Bukry, T. Sato, J. A. Barron, L. A.

Guertin, and R. S. Reese, 2001a, Sequence stratigra-phy of a south Florida carbonate ramp and boundingsiliciclastics (late Miocene Pliocene), in T. M. Mis-simer and T. M. Scott, eds., Geology and hydrology ofLee County, Florida: Florida Geological SurveySpecial Publication 49, p. 3566.

Cunningham, K. J., S. D. Locker, A. C. Hine, D. Bukry,J. A. Barron, and L. A. Guertin, 2001b, Surface-geo-physical characterization of ground-water systems ofthe Caloosahatchee River basin, southern Florida:U. S. Geological Survey Water-resources Investiga-

tions Report 01-4084, 76 p.Cunningham, K. J., S. D. Locker, A. C. Hine, D. Bukry,J. A. Barron, and L. A. Guertin, 2003, Interplay of lateCenozoic siliciclastic supply and carbonate responseon the southeast Florida platform: Journal of Sedi-mentary Research, v. 73, p. 3146.

Cuzella, J. J., C. P. Gough, and S. C. Howard, 1991, Depo-sitional environments and facies analysis of the Cher-okee Group in west-central Kansas, in A. J. Lomandoand P. M. Harris, eds., Mixed carbonate-siliciclasticsequences: SEPM Core Workshop v. 15, p. 273 307.

Eberli, G. P., F. Anselmetti, J. A. M. Kenter, D. F. McNeill,



20/21

R. N. Ginsburg, P. K. Swart, and L. A. Melim, 2001,Facies, diagenesis, and timing of prograding se-quences on western Great Bahama Bank, in R. N.Ginsburg, ed., Subsurface geology of a progradingcarbonate platform margin, Great Bahama Bank:Results of the Bahamas Drilling Project: SEPM SpecialPublication 70, p. 241265.

Erlich, R. N., S. F. Barrett, and G. B. Ju, 1990, Seismic andgeologic characteristics of drowning events on carbon-ate platforms: AAPG Bulletin, v. 74, p. 1523 1537.

Erlich, R. N., A. P. Longo, and S. Hyare, 1993, Response ofcarbonate platform margins to drowning: Evidenceof environmental collapse, in R. G. Loucks and J. F.Sarg, eds., Carbonate sequence stratigraphy, recentdevelopments and applications: AAPG Memoir 57,p. 241266.

Esker, D., G. P. Eberli, and D. F. McNeill, 1998, Thestructural and sedimentologic controls on the reoc-cupation of Quaternary incised valleys, Belize south-ern lagoon: AAPG Bulletin, v. 82, p. 20752109.

Farrell, J. W., S. C. Clemens, and L. P. Gromet, 1995,Improved chronostratigraphic reference curve of lateNeogene seawater 87Sr/86Sr: Geology, v. 23, p. 403406.

Friedman, G. M., 1988, Case histories of coexisting reefsand terrigenous sediments: The Gulf of Elat (RedSea), Java Sea, and Neogene basin of the Negev,Israel, in L. J. Doyle and H. H. Roberts, eds.,Carbonate-clastic transitions: Amsterdam, Elsevier,p. 77 97.

Garca-Mondejar, J., and P. A. Fernandez-Mendiola, 1993,Sequence stratigraphy and systems tracts of a mixedcarbonate and siliciclastic platform-basin setting: TheAlbian of Lunada and Soba, northern Spain: AAPGBulletin, v. 77, p. 245275.

Ginsburg, R. N., 1956, Environmental relationships ofgrain size and constituent composition in somesouth Florida carbonate sediments: AAPG Bulletin,v. 40, p. 23842427.

Ginsburg, R. N., K. M. Browne, and G. S. Chung, 1989,Siliciclastic foundations of south Floridas Quater-nary carbonates (abs.): Geological Society of AmericaAbstracts with Programs, v. 21, p. A-290.

Guertin, L. A., 1998, A late Cenozoic mixed carbonate-siliciclastic system, south Florida: Lithostratigraphy,chronostratigraphy, and sea-level record: Ph.D. dis-sertation, University of Miami, Florida, 424 p.

Guertin, L. A., D. F. McNeill, B. H. Lidz, and K. J.Cunningham, 1999, Biochronology and trangres-

sive/regressive signature in the late Neogene silici-clastic foundation (Long Key Formation) of theFlorida Keys: Journal of Sedimentary Research, v. 69,p. 653666.

Guertin, L. A., T. M. Missimer, and D. F. McNeill, 2000,Correlative sequence boundaries and their hiatalduration from the south Florida platform interior toplatform edge: Chronostratigraphic record of Oligo-cene Pliocene mixed carbonate/siliciclastic sedi-ments: Sedimentary Geology, v. 134, p. 126.

Gvirtzman, G., and B. Buchbinder, 1978, Recent andPleistocene coral reefs and coastal sediments of the

Gulf of Eilat: Post Congress Guidebook, 10th Inter-national Sedimentological Congress, Jerusalem, Is-rael, p. 163 189.

Hallock, P., and W. Schlager, 1986, Nutrient excess andthe demise of coral reefs and carbonate platforms:Palaios, v. 1, p. 389 398.

Handford, C. R., and R. G. Loucks, 1993, Carbonatedepositional sequences and systems tracts Re-sponses of carbonate platforms to relative sea-levelchanges, in R. G. Loucks and J. F. Sarg, eds.,Carbonate sequence stratigraphy, recent develop-ments and applications: AAPG Memoir 57, p. 341.

Haq, B. U., J. Hardenbol, and P. R. Vail, 1987, Thechronology of fluctuating sea level since the Triassic:Science, v. 235, p. 11561167.

Haq, B. U., J. Hardenbol, and P. R. Vail, 1988, Mesozoicand Cenozoic chronostratigraphy and eustatic cycles,in C. K. Wilgus, B. S. Hastings, C. G. St. C. Kendall,H. W. Posamentier, C. A. Ross, and J. C. Van Wagoner,eds., Sea-level changes: An integrated approach: SEPMSpecial Publication 42, p. 71 108.

Hodell, D. A., P. A. Mueller, and J. R. Garrido, 1991, Var-iations in the strontium isotopic composition of sea-water during the Neogene: Geology, v. 19, p. 24 27.

Hoffmeister, J. E., K. W. Stockman, and H. G. Multer,1967, Miami Limestone of Florida and its recentBahamian counterpart: Geological Society of Amer-ica Bulletin, v. 78, p. 175190.

Hoffmeister, J. E., and H. G. Multer, 1968, Geology andorigin of the Florida Keys: Geological Society ofAmerica Bulletin, v. 79, p. 14871502.

Holmes, A. E., and N. Christie-Blick, 1993, Origin of sedi-mentary cycles in mixed carbonate-siliciclastic sys-tems: An example from the Canning Basin, WesternAustralia, in R. G. Loucks and J. F. Sarg, eds., Car-

bonate sequence stratigraphy, recent developmentsand applications: AAPG Memoir 57, p. 181212.James, N. P., 1997, The cool-water carbonate depositional

realm,in N. P. James and J. A. D. Clarke, eds., Cool-water carbonates: SEPM Special Publication 56, p. 1 20.

Kane, B. C., 1984, Origin of the Grandin Sands (Plio-Pleistocene), western Putnam County, Florida: M.S.thesis, University of Florida, Gainesville, 85 p.

Kenter, J. A. M., R. N. Ginsburg, and S. R. Troelstra, 2001,Sea-level driven sedimentation patterns on the slopeand margin, in R. N. Ginsburg, ed., Subsurface geologyof a prograding carbonate platform margin, GreatBahama Bank: Results of theBahamas Drilling Project:

SEPM Special Publication 70, p. 61 100.King Jr., D. T., and H. S. Chafetz, 1983, Tidal-flat toshallow-shelf deposits in the Cap Mountain Lime-stone Member of the Riley Formation, upper Cam-brian of central Texas: Journal of SedimentaryPetrology, v. 53, p. 261273.

Lomando, A. J., and T. L. Walker, 1991, Wamba field,Peoples Republic of Angola, a Cenomanian mixedcarbonate-siliciclastic reservoir,in A. J. Lomando andP. M. Harris, eds., Mixed carbonate-siliciclasticsequences: SEPM Core Workshop v. 15, p. 245271.

Mazzullo, J., A. Malicse, and J. Siegel, 1991, Facies and

42 McNeill et al.


21/21

depositional environments of the Shattuck Sand-stone on the northwest shelf of the Permian Basin:Journal of Sedimentary Petrology, v. 61, p. 940 958.

McKinney, M. L., 1984, Suwannee Channel of thePaleogene coastal plain: Support for the carbonatesuppression model of basin formation: Geology,v. 12, p. 343345.

McNeill, D. F., A. G. Coates, A. F. Budd, and P. F. Borne,2000, Stratigraphy of late Neogene Caribbean reefsand siliciclastics: A coastal emergence record of theIsthmus of Panama: Geological Society of AmericaBulletin, v. 112, p. 963981.

McNeill, D. F., G. P. Eberli, B. H. Lidz, P. K. Swart, andJ. A. M. Kenter, 2001, Chronostratigraphy of prograd-ing carbonate platform margins: A record of sea-levelchanges and dynamic slope sedimentation, westernGreat Bahama Bank, in R. N. Ginsburg, ed., Subsurfacegeology of a prograding carbonate platform margin,Great Bahama Bank: Results of the Bahamas DrillingProject: SEPM Special Publication 70, p. 101134.

Melim, L. A., 1996, Limitations on lowstand meteoricdiagenesis in the PliocenePleistocene of Florida andGreat Bahama Bank: Implications for eustatic sea-level models: Geology, v. 24, p. 893896.

Meyer, F. O., 1989, Siliciclastic influence on Mesozoicplatform development: Baltimore Canyon Trough,western Atlantic, in P. Crevello, J. L. Wilson, J. F.Sarg, and J. F. Read, eds., Controls on carbonateplatform and basin development: SEPM SpecialPublication 44, p. 211232.

Mutti, M., and J. A. Simo, 1993, Stratigraphic patternsand cycle-related diagenesis of upper Yates Forma-tion, Permian, Guadalupe Mountains,in R. G. Loucksand J. F. Sarg, eds., Carbonate sequence stratigraphy,recent developments and applications: AAPG Mem-

oir 57, p. 515534.Oslick, J. S., K. G. Miller, M. D. Feigenson, and J. D.Wright, 1994, Oligocene Miocene strontium iso-topes: Stratigraphic revisions and correlations to aninferred glacioeustatic record: Paleoceanography,v. 9, p. 427443.

Perkins, R. D., 1977, Depositional framework of Pleisto-cene rocks in south Florida, in P. Enos and R. D.Perkins, eds., Quaternary sedimentation in southFlorida: Geological Society of America Memoir 147,p. 131198.

Pirkle, E. C., W. H. Yoho, and A. T. Allen, 1964, Origin ofthe silica sand deposits of the Lake Wales Ridge areaof Florida: Economic Geology, v. 59, p. 11071139.

Playford, P. E., N. F. Hurley, C. Kerans, and M. F.Middleton, 1989, Reefal platform development, De-vonian of the Canning Basin, Western Australia, inP. Crevello, J. L. Wilson, J. F. Sarg, and J. F. Read, eds.,Controls on carbonate platform and basin develop-ment: SEPM Special Publication 44, p. 187202.

Puri, H. S., and R. O. Vernon, 1964, Summary of thegeology of Florida and a guidebook to the classicexposures: Florida Geological Survey Special Publica-tion 5, 312 p.

Roberts, H. H., and S. P. Murray, 1988, Gulf of thenorthern Red Sea: Depositional setting of abrupt

siliciclastic-carbonate transitions, in L. J. Doyle andH. H. Roberts, eds., Carbonate-clastic transitions:Amsterdam, Elsevier, p. 99142.

Santisteban, C., and C. Taberner, 1988, Sedimentary mod-els of siliciclastic deposits and coral reef interrela-tion,in L. J. Doyle and H. H. Roberts, eds., Carbonate-clastic transitions: Amsterdam, Elsevier, p. 3576.

Schlager, W., 1989, Drowning unconformities on carbon-ate platforms, in P. Crevello, J. L. Wilson, J. F. Sarg,and J. F. Read, eds., Controls on carbonate platformand basin development: SEPM Special Publication 44,p. 15 25.

Schlager, W., and O. Camber, 1986, Submarine slopeangles, drowning unconformities, and self-erosion oflimestone escarpments: Geology, v. 14, p. 762765.

Scott, T. M., 1988, The lithostratigraphy of the HawthornGroup (Miocene) of Florida: Florida GeologicalSurvey Bulletin 59, 148 p.

Shinn, E. A., B. H. Lidz, J. L. Kindinger, J. H. Hudson, andR. B. Halley, 1989, Reefs of Florida and the DryTortugas, a guide to the modern carbonate environ-ments of the Florida Keys and the Dry Tortugas: FieldTrip Guidebook T176 for the 28th InternationalGeological Congress, Washington, D.C., AmericanGeophysical Union, 55 p.

Simo, A., 1989, Upper Cretaceous platform-to-basindepositional-sequence development, Tremp Basin,south-central Pyrenees, Spain, in P. Crevello, J. L.Wilson, J. F. Sarg, and J. F. Read, eds., Controls oncarbonate platform and basin development: SEPMSpecial Publication 44, p. 365399.

Smosna, R., and D. G. Patchen, 1991, Ordovician lime-stone and shale in the central Appalachian Basin:Early sedimentary response to plate collision,in A. J.Lomando and P. M. Harris, eds., Mixed carbonate-

siliciclastic sequences: SEPM Core Workshop v. 15,p. 485509.Southgate, P. N., J. M. Kennard, M. J. Jackson, P.E. OBrien,

and M. J. Sexton, 1993, Reciprocal lowstand clasticand highstand carbonate sedimentation, subsurfaceDevonian reef complex, Canning Basin, WesternAustralia, in R. G. Loucksand J. F. Sarg, eds., Carbonatesequence stratigraphy, recent developments andapplications: AAPG Memoir 57, p. 157 179.

Stanley, S. M., 1966, Paleoecology and diagenesis of KeyLargo Limestone, Florida: AAPG Bulletin, v. 50,p. 1927 1947.

Van Wagoner, J. C., R. M. Mitchum, K. M. Campion, andV. D. Rahmanian, 1990, Siliciclastic sequence stratig-

raphy in well logs, cores, and outcrops: Concepts forhigh-resolution correlation of time and facies: AAPGMethods in Exploration Series 7, 55 p.

Vaughan, T. W., 1910, A contribution to the geologichistory of the Floridian Plateau: Papers from theTortugas Laboratory, v. IV: Carnegie Institution ofWashington Special Publication 133, p. 99185.

Warzeski, E. R., K. J. Cunningham, R. N. Ginsburg, J. B.Anderson, and Z.-D. Ding, 1996, A Neogene mixedsiliciclastic and carbonate foundation for the Quater-nary carbonate shelf, Florida Keys: Journal of Sedi-mentary Research, v. 66, p. 788800.


Documents

Chapter 2 AAPG