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Palaeogeography, Palaeoclimatology, Palaeoecology 210 (2004) 367–384

Cherty carbonate facies of the Montoya Group, southern New

Mexico and western Texas and its regional correlatives: a record of

Late Ordovician paleoceanography on southern Laurentia

Michael C. Pope*

Department of Geology, Washington State University, Pullman, WA 99164-2812, USA

Received 25 November 2002; accepted 23 February 2004

Abstract

The Upper Ordovician Montoya Group in southern New Mexico and westernmost Texas records predominantly subtidal

deposition on a gently dipping carbonate ramp that was subsequently nearly entirely dolomitized. The medial unit of the

Montoya Group, the Aleman Formation is unique because it contains abundant chert (10–70% by volume). The chert occurs as:

(1) thin continuous beds of sponge spicules within mudstone or calcisiltite; (2) discontinuous, lenses or nodules within skeletal

wackestones and packstones; or (3) as a replacement of skeletal grains and burrows. Coeval skeletal grainstones and muddy

peritidal facies contain little chert. Phosphate (up to 5 wt.%) occurs within the underlying Upham Formation and the Aleman

Formation as replacement of fossils and peloids. The abundance of chert and phosphate in these subtidal facies indicates they

formed within a region of strong upwelling. Regional correlation with Upper Ordovician cherty units along the periphery of

southern Laurentia and other low latitude continents suggests that upwelling was widespread and long-lived during the Late

Ordovician. The upwelling is interpreted to record vigorous oceanic circulation produced by the onset of glaciation on

Gondwana during this period. Late Ordovician relative sea-level curves around the periphery of Laurentia indicate correlative

third-order (1–3 my duration) fluctuations that may provide a means for high-resolution global correlations. However, the

mechanism(s) that produced these long-term fluctuations are unclear.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Upwelling; Glaciation; Chert; Phosphate; Eustasy

1. Introduction

Dropstones, diamictites, and striated pavements in

northern Africa record a Late Ordovician glacial event

(Crowell, 1999). This glaciation is enigmatic because

it formed during a prolonged period of global green-

0031-0182/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.palaeo.2004.02.035

* Fax: +1-509-335-5989.

E-mail address: [email protected] (M.C. Pope).

house conditions in which atmospheric pCO2 was an

estimated 10–18 times greater than present (Berner,

1994). Glaciation was likely triggered by the migra-

tion of Gondwana across the South Pole (Crowley and

Baum, 1991, 1995), possibly in conjunction with a

global decrease in atmospheric temperature driven by

intense silicate weathering of Taconic orogenic high-

lands (Kump et al., 1999). A 3–7x positive excur-

sion in both d13C and d18O stable isotopes of

carbonates, and a record of widespread sea-level

Fig. 1. Late Ordovician global map modified from Scotese (1997). L= Laurentia, B =Baltica, S = Siberia, and C =China. Montoya Group

locality marked by black circle. The location of cherty carbonates is approximate, see references in Pope and Steffen (2003). Paleoceanographic

currents modified from Pope and Read (1998).

Fig. 2. Location map of Montoya Group outcrops in southern New Mexico and western Texas. Generalized Montoya Group sequence

stratigraphic cross-section (Fig. 5) occurs along B–BV.

M.C. Pope / Palaeogeography, Palaeoclimatology, Palaeoecology 210 (2004) 367–384368

M.C. Pope / Palaeogeography, Palaeoclimatology, Palaeoecology 210 (2004) 367–384 369

drawdown from three continents during the Hirnan-

tian Stage is interpreted as evidence for a short-lived,

possibly < 1 million-year-long glaciation (Brenchley

et al., 1994; Marshall et al., 1997). A longer glacial

event is indicated by Late Middle–Late Ordovician

glaciogenic sedimentary rocks in Africa (Barnes,

1986; Frakes et al., 1992; Theron, 1994; Crowell,

1999). However, the physical and paleontologic re-

cord for the duration of this glaciation is equivocal.

High-frequency, moderate amplitude (20–30 m/20–

100 ky) sea-level fluctuations in the Upper Ordovician

Lexington Limestone of the Appalachian Basin were

interpreted to be indirect proxy evidence for a pro-

longed Ordovician glaciation (Pope and Read, 1997a,

1998). This paper discusses new sedimentologic and

stratigraphic data from the Upper Ordovician Mon-

toya Group of Texas and southern New Mexico and

their regional equivalents that indicate the southern

margin of Laurentia was the locus of intensive up-

welling. Enhanced thermohaline circulation caused by

the Gondwana glaciation is postulated to have pro-

duced this long-lived, widespread upwelling zone.

Fig. 3. Stratigraphic chart of Montoya Group and its correlatives along th

area given in the text. Dark gray stippling indicates cherty carbonate unit

upwelling around North America is indicated by the extent of cherty carbon

U–Pb dating of the Diecke bentonite in the US Midcontinent (Tucker et a

442 Ma (Tucker and McKerrow, 1995).

Long-term relative sea-level fluctuations recorded in

the Montoya Group are correlative with similar events

around Laurentia and may provide a means for high-

resolution stratigraphic correlation. The mechanism(s)

that produced these long-term relative sea-level fluc-

tuations are unclear.

2. Regional stratigraphic framework

Upper Ordovician Montoya Group strata in New

Mexico and western Texas are up to 180 m thick and

formed on a gently sloping carbonate ramp on the

Laurentian passive margin (Measures, 1985a,b; LeM-

one, 1988). Paleomagnetic reconstructions indicate

this passive margin (Fig. 1) was positioned between

5j and 20j south latitude, aligned approximately

east–west and faced the Panthalassic Ocean (Scotese,

1997; Mac Niocaill et al., 1997).

The Montoya Group outcrops in uplifted, block-

faulted mountain ranges throughout southern New

Mexico and westernmost Texas that are surrounded

e southern margin of Laurentia. Biostratigraphic references for each

s. P indicates units containing abundant phosphate. The duration of

ate. The Turinian–Chatfieldian boundary occurs at f 454 based on

l., 1990), whereas the Ordovician–Silurian boundary is estimated at


by flat-floored valleys (Fig. 2). These mountains were

produced by Cenozoic basin and range extension and

they trend north–northwesterly. Forty-two full and

partial outcrop measured sections (Fig. 2) and over

300 standard thin sections were used for analysis of

these rocks.

Extensive studies of the Montoya Group by strat-

igraphers and paleontologists (e.g. Kelley and Silver,

1952; Pray, 1958; Hill, 1959; Howe, 1959; Flower,

1961, 1969; Pratt and Jones, 1961; Kottlowski, 1963;

LeMone, 1969; Hayes, 1975; Sweet, 1979) produced

the gross regional lithostratigraphy (Fig. 3) that

remains in use today. These studies indicate that the

Montoya Group was deposited during the Cincinna-

tian Series (Late Ordovician) and that the latest

Ordovician Hirnantian Stage was a time of erosion

or non-deposition in this area (Fig. 3).

Fig. 4. Cross-section diagrams showing the distribution of facies across th

HST. During the TST siliciclastics of the inner part of the ramp are pr

grainstone then deep ramp interbedded calcisiltite or mudstone and spiculi

by the initial deep ramp calcisiltite and mudstone interbedded with spicu

packstone then skeletal packstone and grainstone. Burrowed skeletal wac

were deposited updip, behind the skeletal grainstone and packstone.

The Montoya Group in southern New Mexico and

westernmost Texas is subdivided into three formations

(Fig. 3) given in ascending order, Upham, Aleman,

and Cutter. All of these units are extensively dolomi-

tized with limestone occurring locally only in the

Upham and basal Aleman. The Upham Formation

(Fig. 4) conformably overlies a basal, thin (up to 16 m

thick), calcareous-cemented sandstone and gravel

conglomerate (Cable Canyon Sandstone) that rests

unconformably above the Lower Ordovician El Paso

Group. The Upham Formation typically is a light to

dark colored, bioturbated wackestone to grainstone

approximately 13–42 m thick. This unit has a grada-

tional contact with the underlying sandstone or lies

disconformably on the El Paso Group. The Aleman

Formation (16–85 m thick) conformably overlies the

Upham Formation and generally consists of a lower,

e Montoya Group carbonate ramp during the second-order TST and

ogressively overlain by mid-ramp burrowed skeletal packstone or

tic chert. The HST is a shallowing-upward succession characterized

litic chert grading upward into mid-ramp skeletal wackestone and

kestone and mudstone, laminated mudstone and fenestral mudstone


intercalated dark brown-gray, thin-bedded carbonate

and chert unit that grades up into nodular, cherty

dolostone (Fig. 4). The Cutter Formation (30–60 m

thick), composed of light colored, fine-grained dolo-

mite with little chert, is the uppermost unit of the

Montoya Group (Fig. 4). A regional unconformity

separates the Montoya Group from the overlying

Silurian Fusselman Dolomite or younger units.

3. Generalized sequence stratigraphy

The Montoya Group measured sections were ana-

lyzed using sequence stratigraphic principles outlined

in Harris et al. (1999). The Montoya Group was

deposited on a carbonate ramp (sensu Ahr, 1973)

because shallow to deep water facies pass laterally

into one another (Fig. 4) without evidence for a

substantial change in depositional slope (Read, 1985).

A generalized outline of the Montoya Group

sequence stratigraphy (Fig. 5) is presented here to

facilitate the following discussions of: (1) the region-

al stratigraphic position and distribution of cherty

facies on this ramp; and (2) the correlative third-order

(1–3 my) sea-level changes recorded in these units.

The sequence stratigraphic terminology in this paper

Fig. 5. Generalized sequence stratigraphy along Line B–BV from Fig. 2. Th

flooding zone of cherty carbonate (Pope et al., 2001). The cherty carbonates

follows the hierarchy of Weber et al. (1995) for

naming sequence stratigraphic depositional units.

The generalized sequence stratigraphic framework

of the Montoya Group sequence relies on Line B–

BV (Fig. 2). This generalized sequence stratigraphic

analysis indicates the Montoya Group is a single

second-order (8–10 my duration) transgressive to

regressive supersequence composed of six regionally

correlative third-order (1–3 my duration) sequences

(Pope et al., 2001). The third-order sequences are

numbered sequentially upward (0–5) from the base.

Sequences 0 and 1 have quartz, sandy bases that are

overlain by subtidal, mid- to shallow-ramp biotur-

bated skeletal wackestone/packstone. Updip, these

sequences are capped by high-energy skeletal grain-

stone. Sequence 2 has chert-rich, deep ramp carbon-

ate at its base and its upper part is marked by

progradation of a high-energy shallow ramp skeletal

grainstone with abundant colonial corals. Muddy

peritidal carbonates were deposited behind the skel-

etal shoal. Sequence 3 is comprised of an aggrada-

tional stack of skeletal packstone with colonial corals

that separates muddy inner ramp peritidal facies from

deeper ramp cherty carbonates. Sequence 4 is marked

by a pronounced basinward shift of peritidal facies

into the basin. Sequence 5 is only preserved in

e second-order systems tracts are separated by a regional maximum

of the Aleman Formation occur within third-order sequences 2 and 3.


downdip locations and is comprised of coarse skel-

etal grainstone.

Fig. 6. (A) Photo of even-bedded calcisiltite and spiculitic chert,

lower Aleman Formation, Cooks Range, New Mexico. Ten-

centimeter black scale bar in lower right center of photo. (B) Photo

of nodular chert in burrowed skeletal wackestone and packstone,

north Franklin Mountains, Texas. (C) Silicified brachiopods in

skeletal packstone, Caballos Mountains, New Mexico. Five-

centimeter scale bar.

4. Cherty facies of the Aleman Formation

The cherty facies of the Aleman Formation are

shown graphically in Fig. 4. The Aleman Formation

is a complex subtidal carbonate unit containing

abundant chert (10–70% by volume). The Aleman

Formation commonly is subdivided into upper and

lower cherty units that are separated by a widespread

medial packstone/grainstone marker unit (Fig. 5).

The depositional environments represented by the

Aleman Formation range from shallow, high-energy

shoals to deep-water settings, below storm wave base

(Fig. 4).

4.1. Even-bedded laminated calcisiltite or mudstone

and spiculitic chert

Even-bedded laminated calcisiltite and spiculitic

chert (Fig. 6A) is the basal unit of the Aleman

Formation. The calcisiltite generally is horizontally

laminated but locally shows small hummocky cross-

lamination. The chert is composed almost entirely

(>90%) of sponge spicules that were cemented by

later silica or carbonate. Locally the spiculitic chert

is interbedded with massive mudstone and this facies

is most common in the base of the lower part of the

upper Aleman above the medial packstone/grain-

stone marker. The even-bedded calcisiltite or mud-

stone commonly contains 1–5 wt.% phosphate as

pellets, with rare phosphatic sponge spicules occur-

ring in the interbedded spiculitic chert. Locally

within the even-bedded calcisiltite and spiculitic

chert are beds 1–3 m thick of cross-laminated

spiculite up to 3 m thick.

The even bedded calcisiltite or mudstone inter-

bedded with spiculitic chert is interpreted to repre-

sent deposition in deep waters commonly below

storm wave base. The calcisiltite and mudstone

likely represent the background sedimentation on

this ramp, whereas the spiculitic chert formed as the

disarticulated sponge spicules were redistributed by

storms or currents and accumulated. It is unclear

what caused the alternation of carbonate and silica,

the periods of abundant silica deposition may record

higher frequency climate or oceanographic changes

(e.g. Elrick et al., 1991). The abundance of sponge

spicules and absence of any other fauna, save small

brachiopods, indicates this facies may have formed

in cool or oxygen-poor waters (for example, James,

1997). The hummocky beds within this facies

indicate that storm wave base did sometimes im-

M.C. Pope / Palaeogeography, Palaeoclimatol

pinge upon the seafloor during deposition of this

facies.

4.2. Skeletal wackestone to packstone with irregular,

discontinuous bedded to nodular chert

Skeletal wackestone to packstone containing irreg-

ular and discontinuously bedded chert up to a few

meters wide and a few cm thick on outcrop grades

laterally and vertically into skeletal wackestone/pack-

stone with nodular bedded chert (Fig. 6B). This facies

occurs within both the lower and upper Aleman

Formation, seaward of the grainstone shoal complex

and above the interbedded calcisiltite or mudstone and

spiculitic chert (Fig. 4). The abundance of chert in this

facies varies from 5–60%. The chert nodules range

from a few cm’s to tens of cm’s in diameter. The chert

margins vary from smooth to sharp and irregular.

Some chert nodules contain carbonate within their

centers giving them a ‘‘hollow’’ appearance (see

Howe, 1959). Primary laminations within the skeletal

wackestone/packstone are rare, but where they occur

with the chert, the laminations commonly are bent

around the nodules. The nodular chert occurs primarily

in skeletal wackestone to skeletal packstone. Sponge

spicules occur in many of the silicified nodules but

their abundance commonly is much less than in the

interbedded calcisiltite and spiculite facies. Phosphate

occurs throughout this facies as peloids, coatings on

hardgrounds and as a replacement of skeletal grains,

most commonly of bryozoans. Additionally, there are

many silicified burrows and fossils (Fig. 6C) within

the nodular cherty wackestone/packstone. Brachio-

pods are the main skeletal grains in this facies with

lesser amounts of bryozoan and crinoid fragments.

These cherty nodular carbonate facies formed on

an open marine ramp. The variety of chert abundance

and morphologies reflects both original depositional

features and subsequent early diagenetic silica enrich-

ment. The abundance of brachiopods and lack of other

warm-water organisms (corals, green algae) suggest

this facies formed in cool waters. The lack of bedding

and nodular appearance of chert suggests this facies

was intensely bioturbated. Deformed lamination sur-

rounding chert nodules, silicification of burrows and

unflattened skeletal fragments indicate that much of

the silica in this facies formed prior to burial and

compaction.

4.3. Skeletal packstone/grainstone

Skeletal packstone/grainstone including common

crinoids, bryozoans, brachiopods and rare rugose

corals and stromatoporoids is interbedded with thin

beds of colonial tabulate coral bafflestone comprises

widespread marker unit in the middle of the Aleman

Formation (Howe, 1959). This unit developed be-

tween upramp peritidal mudstone and downramp

subtidal carbonates with abundant chert (Fig. 5). This

facies commonly contains little silica, except where

the colonial corals commonly are replaced by chert.

Low-angle tangential cross-bedding locally is com-

mon within this facies. Hardground surfaces also are

common within this facies and these locally are

encrusted by phosphate.

The skeletal packstone/grainstone and coral baffle-

stone is interpreted to represent a high-energy skeletal

shoal or coral thicket. Widespread cross-bedding in

this unit indicates high-energy currents during depo-

sition of this unit. The abundance of corals in this

facies indicates that they were deposited in warm,

normal marine waters.

ogy, Palaeoecology 210 (2004) 367–384 373

5. Types of chert in Montoya Group

The chert in the Montoya Group is divided into

three types: primary, early diagenetic and late diage-

netic. The nature of contacts between the chert and

carbonate within an individual bed varies greatly from

gradational to sharp. Many of the chert beds contain

small euhedral dolomite crystals formed after forma-

tion of the chert.

5.1. Primary depositional chert

Centimeter-thick beds of chert interbedded with

calcisiltite or mudstone and elongate discontinuous

chert lenses in calcisiltite or mudstone are considered

primary depositional because petrography and etching

of samples with HF reveals that they are composed

almost entirely of sponge spicules. Similarly, cross-

bedding in these spiculites indicate either storm re-

working or downslope current transportation of abun-

dant spicules. These chert beds formed from an

accumulation of sponge spicules on the seafloor, and

the lack of abundant storm features (i.e. graded


bedding, hummocky cross stratification) suggests they

commonly formed below storm wave base.

5.2. Early diagenetic chert

Almost all the nodular and irregular chert nodules

in the Aleman Formation are considered early dia-

genetic because laminae in the carbonate sediment

are bent around the chert. Also, many whole, uncom-

pacted fossils and undeformed burrows in the Ale-

man Formation are silicified suggesting this chert

formed early on the sea floor, or immediately below

the sediment–water interface, prior to complete

lithification.

5.3. Late diagenetic chert

Late diagenetic chert commonly occurs in three

forms. (1) White to light gray nodules that cross-cut

bedding and occur primarily in tidal flat facies are

interpreted to be a replacement of evaporite nodules.

(2) Gray to white nodules within subtidal facies cross-

cut bedding of dark gray carbonate containing early

diagenetic chert (e.g. Geeslin and Chafetz, 1982)

occur primarily updip and are also interpreted to be

a replacement of evaporites. However, these evapor-

ites likely formed from burial brines during subse-

quent exposure of the Montoya carbonate platform.

(3) Elongate veins, or tabular beds that cross cut or

parallel bedding.

6. Regional correlations

The Montoya Group is regionally correlative with

Upper Ordovician cherty and phosphatic carbonates

of the southern Midcontinent and Appalachian Basin

(Fig. 3). These correlations are described here to

determine the paleoceanographic significance of these

units.

The Trenton Group (Upper Ordovician) and its

equivalents (Galena Group, Lexington Limestone) in

the Appalachian Basin and northern midcontinent are

shallow-water carbonates that were deposited in cool,

phosphate-rich waters (Brookfield, 1988; Lavoie,

1995; Patzkowsky and Holland, 1996; Pope and

Read, 1997a, 1998; Holland and Patzkowsky, 1997;

Kolata et al., 2001). These cool-water carbonates are

conformably overlain by Cincinnatian shaly cool-

water carbonates (Holland, 1993; Pope and Read,

1997b; Holland and Patzkowsky, 1997). As the Ap-

palachian Basin filled during the Cincinnatian, fine

siliciclastics prograded progressively farther to the

west and south as that basin was filled (Kolata et

al., 2001).

The Viola Group of Oklahoma has two parts, a

lower Trenton Group (Upper Ordovician) equivalent

that is unconformably overlain by Cincinnatian strata

(O’Brien and Derby, 1997). The Viola Group consists

of interbedded carbonate and chert deposited on a

steep ramp that includes contourites and turbidites

(Brown and Sentfle, 1997). The basal part of the Viola

Group is particularly siliceous (up to 70% chert),

including both biogenic (sponge spicules) and sec-

ondary silica (Galvin, 1983; Candelaria and Roux,

1997). The lower cherty carbonate of the Viola Group

is comprised of nodular-bedded skeletal wackestone/

packstone with light brown-gray chert nodules (Fig.

7). The nodular bedding was likely produced by a

combination of burrowing and uneven marine precip-

itation, followed by subsequent uneven compaction.

The chert is interpreted to be early diagenetic since

bedding in this facies is deformed around the chert.

Karst within the Viola Group in Oklahoma formed

prior to the Richmondian (Sykes et al., 1997) and may

be correlative to shallowing and progradation of

grainstone within the Aleman Formation. The top part

of carbonate part of the Viola Group is a crinoidal

grainstone (equivalent to Fernvale, Welling) that is

conformably overlain by the Richmondian Sylvan

Shale. This grainstone may be equivalent to the

grainstone that prograded near the top of Sequence

2 in the Montoya Group. Structural differentiation of

basins and uplifts segmented the southern Oklahoma

part of the Ouachita basin to produce distinctive

lithologies in each area (Denison, 1997). The basins

commonly are muddier and chert-rich, whereas the

uplifts are grain-rich, preferentially dolomitized, and

contain abundant hardgrounds, more karstic surfaces,

and less chert. Locally the Viola Group contains

sandstone beds such as First Wilcox Sand that were

shed off subtle highs in Arkansas and Oklahoma

during high-frequency sea-level falls (see O’Brien

and Derby, 1997; Denison, 1997). Organic-rich shale

and chert in the Big Fork Chert and Polk Creek Shale

units (Finney, 1986) occurs outboard of the Viola

Fig. 7. Photo of nodular carbonate and light-colored chert in lower Viola Group, I-35, Ardmore County, Oklahoma. Ten-centimeter scale bar in

lower middle of photo.


Group cherty-carbonate facies and indicates this area

of the margin received abundant dissolved silica

throughout the Late Ordovician.

The Richmondian Sylvan Shale is a poorly ex-

posed unit that overlies the top of the Viola Group and

underlies the latest Ordovician (Gamachian–Hirnan-

tian) Keel Oolite. The Sylvan Shale records a single

shallowing-upward trend of graptolitic black shale

passing upward into unfossiliferous gray and green

shale (Finney, 1988). The water depths of the Sylvan

are unknown, but it is not entirely a deepwater unit,

rather it reflects muddying of the waters which poi-

soned the carbonate system shutting down the car-

bonate factory (Amsden, 1988). This scenario is

compatible with the onset of a gradual long-term

sea-level drop that begins in the Cincinnatian and

induced progradation of fine siliciclastics to the west

and south from the Appalachian Basin (Kolata et al.,

2001).

The Hirnantian Keel Oolite is part of a thin, locally

developed succession of iron and aragonitic ooids that

occur discontinuously in the US Midcontinent and

southern Canada (see Sharma and Dix, this volume).

These units formed during the sea-level drawdown

likely associated with the maximum extent of Late

Ordovician glaciation (Brenchley et al., 1994).

The Maravillas Formation in the Marathon Uplift

of south-central Texas consists of interbedded fine-

grained, organic-rich chert and carbonate deposited

in a deep subtidal setting (McBride, 1969, 1970,

1989). The Maravillas Formation was deposited

during the Late Ordovician (Goldman et al., 1995).

The lower part of the Maravillas Formation is a

succession, up to 130 m thick of interbedded thin to

thick beds of carbonate and chert (Fig. 8A). The

carbonate beds (5 cm to 2 m thick) are calcisiltite and

carbonate-clast conglomerate which commonly fine

upward (Fig. 8B). The carbonate units were deposited

by carbonate turbidity currents (McBride, 1969,

1970) whose presence suggests the Maravillas For-

mation was deposited in a tectonically active setting,

or below a break in the ramp slope, possibly on a

distally steepened ramp. The chert morphologies in

the Maravillas Formation are quite varied from even,

uniformly dark homogeneous beds to undulatory beds

and less common chert nodules. The Maravillas

Formation chert contains abundant sponge spicules,

phosphate and glauconite (McBride, 1969, 1989).

The chert of the Maravillas Formation is interpreted

to have formed during deposition and early diagene-

sis. The upper 20–30 m of the Maravillas Formation

consists of cherty shale or shale (McBride, 1969,

1970) that may be correlative with the Sylvan Shale

in Oklahoma. The Maravillas Formation was thrusted

northward to its present position during the Late

Paleozoic, indicating the Ordovician carbonate ramp

Fig. 8. (A) Photo of interbedded carbonate and chert the Maravillas Formation along Texas State Highway 385, approximately 15 km south of

Marathon, Texas. Person for scale is approximately 1.7 m tall. (B) Close-up of normal graded carbonate conglomerate between black chert and

light gray lime mudstone in Maravillas Formation.


upon which it formed was much wider than its

current configuration.

Biostratigraphic correlation indicates the Montoya

Group, Maravillas Formation, and Viola Springs

Group formed a laterally continuous zone of cherty

carbonates across the southern Laurentia margin dur-

ing the Late Ordovician (Fig. 9). Similarly, Upper

Ordovician shelf carbonates in Sonora, Mexico pass

downramp into interbedded chert and carbonate then

into basinal chert and shale (Poole et al., 1995a,b),

suggesting this belt of cherty carbonates was contin-

uous from Oklahoma into northern Mexico. The

possibility of many hundreds of kilometers of Meso-

zoic left-lateral offset along this margin (Silver and

Anderson, 1974; Anderson and Silver, 1979) does not

negate the correlation, just its relative position.

Upper Ordovician cherty carbonates are not re-

stricted to southern Laurentia, but also occur discon-

tinuously in the North American Cordillera from the

Great Basin to northern Canada and Alaska (Figs. 1

and 9). In these areas, interbedded chert and grapto-

litic shale were deposited in deep water seaward of

cherty carbonates that pass upramp into peritidal

carbonates (Ross, 1976; Miller, 1975, 1976).

Coeval interbedded chert and shale or cherty shale

units were also deposited in low-latitude settings on

Baltica, Siberia, New Zealand and Australia (Fig. 1).

The shale in these units commonly is organic-rich,

Fig. 9. Map showing evidence for late Middle to Late Ordovician upwelling around the US. Upwelling is marked in the Appalachian Basin by

cool-water carbonates (Brookfield, 1988; Patzkowsky and Holland, 1993; Lavoie, 1995; Pope and Read, 1997a, 1998) containing abundant

phosphate (up to 10 wt.%) along the Cincinnati Arch (Holland and Patzkowsky, 1997; Pope and Read, 1997a, b). The Sebree Trough funneled

cool, deep oceanic waters onto the US Midcontinent (Kolata et al., 2001). Warm water tidal flats only developed in the Late Ordovician, updip

along the western and northern limit of outcrops. Cherty carbonates described in this paper occur along the southern margin of Laurentia from

Oklahoma to Mexico. Chert-rich carbonate facies occur in the subsurface data from the Permian basin, Texas. Outboard of the cherty carbonates

are cherty shales. Cherty carbonates and cherty shale also occur in the southern Great Basin and Idaho.


and may contain abundant phosphate and numerous

graptolites. The chert in these units commonly occurs

as nodules containing radiolarians or sponge spicules.

Silicified nodules within the shaly units are interpreted

to have formed when sea-level was high (e.g. Loi and

Dabard, 2002), similar to the model for formation of

Aleman Formation chert nodules proposed below.

7. Discussion

7.1. Upwelling in the Late Ordovician

The abundance of stromatoporoids, corals, and

receptacularid algae (LeMone, 1969, 1988) in the

Upham Formation indicates that this unit was depos-

ited in warm oceanic waters. Similarly, the occurrence

of corals in the shallow subtidal facies of the Aleman

Formation indicates that these rocks also formed in

warm oceanic waters. However, the deeper subtidal

portions of the Aleman are dominated by siliceous

sponge spicules and brachiopods indicating this is a

heterozoan fauna (James, 1997). The brachiopods in

the Aleman Formation and deeper subtidal portions of

the Cutter Formation (Howe, 1959) contain many of

the genuses (Hebertella, Platystrophia, Rafinesquina)

interpreted to define a cool water biofacies in the

Appalachian Basin (Patzkowsky and Holland, 1999).

Additionally, the lack of ooids, peloids, hardgrounds

and calcareous algae in these rocks also may indicate

they formed in cooler oceanic waters (cf. Patzkowsky

and Holland, 1993; Pope and Read, 1997a,b). The

mudcracked and bioturbated dolomudsone with occa-

sional corals of the peritidal facies in the Cutter

Formation suggest the updip part of the Montoya

Group was deposited in warm oceanic waters. Thus,


it appears the upper part of the Montoya Group may

have been deposited on a temperature-stratified ramp

with cool, deeper subtidal waters passing updip into

warm, shallow subtidal and peritidal waters. This

depositional model is very similar to one put forth

to explain the coeval juxtaposition of subtidal cool-

water and peritidal warm-water Late Ordovician car-

bonates in the US Midcontinent (Kolata et al., 2001)

and the Appalachian Basin (Pope and Read, 1998).

The abundance of spiculitic chert, early diagenetic

chert and phosphate (1–5 wt.%) in the Montoya

Group indicates these formed in an upwelling zone

(e.g., Parrish, 1998). Direct correlations and similar-

ities between the Montoya Group, Maravillas Forma-

tion, Viola Group and unnamed Upper Ordovician

units in Mexico indicates the upwelling zone was

widespread along the southern margin of Laurentia

(Fig. 9). Global sea-level was at, or near, its Paleozoic

maximum during the Late Ordovician and most of

North America was submerged and could not have

been a substantial source of silica. Thus, the silica and

phosphate in the Upper Ordovician cherty carbonates

of southern Laurentia were brought into this area by

upwelling of cool, deeper oceanic waters. Biostratig-

raphy (Fig. 3) suggests the upwelling lasted up to 10–

12 my (454–442 Ma).

In modern oceans, coastal upwelling occurs pri-

marily along the western margins of low–middle

latitude continents where surface winds blowing par-

allel to the coast, and slightly offshore push surface

waters through Ekman transport perpendicular to the

shoreline. This offshore movement of water is bal-

anced through replacement from below by cool,

nutrient rich waters. These upwelling waters generally

come from the upper few hundred meters of the

ocean. These upwelling zones are characterized by

abundant phosphate, silica, and organic-rich shale

(Parrish, 1998). The appearance of widespread and

anomalously cherty and phosphatic carbonates in the

Late Ordovician of Laurentia indicates that strong

equatorial upwelling began during this period. Unlike

modern upwelling the distribution of cool water,

phosphate-, and silica-rich carbonates occurs on mar-

gins that were perpendicular to prevailing winds (Figs.

1 and 9), especially on the US Midcontinent, and

possibly along southern Laurentia.

Upper Ordovician bedded carbonate, chert, and

phosphate deposits of southern Laurentia and the

North American midcontinent are anomalous in the

Paleozoic because they represent the influx of cool

oceanic waters, with abundant silica and phosphate,

hundreds of kilometers onto the interior of this con-

tinent (Lavoie, 1995; Patzkowsky and Holland, 1996;

Pope and Read, 1998; Kolata et al., 2001). Interbed-

ded chert and shale commonly were deposited ocean-

ward of the cherty carbonates (e.g. Big Fork chert,

upper Maravillas Formation shaly chert, unnamed

units in Mexico). Graptolitic-rich shales may mark

the position of upwelling along the shelf margin (e.g.

Finney and Berry, 1997) and coeval cherty carbonates

inboard and graptolitic shale and chert outboard are

consistent with laterally extensive (up to a few hun-

dred kilometers wide) upwelling across the southern

Laurentia margin. The development of this upwelling

zone corresponds with a northward expansion of cool

water trilobite faunas (Shaw, 1991) and a pronounced

shift to cooler-water benthic faunas across eastern

North America (Patzkowsky and Holland, 1993).

Similarly, the abundance of silica-replaced fossils

and bedded chert throughout the Late Ordovician

(Kidder and Erwin, 2001; Pope and Steffen, 2003)

suggests a substantial global increase in upwelling

during this period. The abundance of Late Ordovician

cherty carbonate and cherty shale units and the dearth

of these units in the Middle Ordovician and during the

Early Silurian suggests a global climatic or oceano-

graphic origin for these deposits.

Glacially mixed oceans commonly are marked by

the development of widespread equatorial upwelling

zones, whereas greenhouse oceans generally lack

widespread upwelling zones (Hay, 1988). Similarly,

the occurrence and abundance of widespread phos-

phorites commonly indicates well-mixed oceans (e.g.

Berner, 1996). Thus, the coupled origination of geo-

graphically widespread equatorial or near-equatorial

upwelling and phosphogenesis in the Late Ordovician

may reflect the initiation of Gondwana glaciation. The

initiation of this upwelling in the Late Ordovician

corresponds with cool (13–19 jC) surface waters in

the Appalachian Basin (Railsback et al., 1990), and

the approximate initiation of glaciogenic deposits in

Africa (Theron, 1994). The presence of geographical-

ly widespread upwelling zones developed hundreds of

kilometers onto Laurentia over a 10–12 my period

also is in accord in accord with computer modeling,

which indicates that there was increased poleward


heat flow during the Late Ordovician glaciation

(Poussart et al., 1999).

7.2. Relative sea-level record from North America

The Late Ordovician is a time of high sea-level

around North America following a global sea-level

low that occurred during the Middle Ordovician

(Schutter, 1992). The six, regionally correlative de-

positional sequences in the Montoya Group record

long-term (1–3 my) changes in relative water-depth

in this area during the later Late Ordovician (Fig. 10).

The North American data indicates a long-term

Fig. 10. Correlation diagram of published late Middle to Late Ordovician s

North America. Maximum water depth, marked with a black circle, approx

Montoya curve shows six third-order (1–3 my duration) relative sea-level r

relative sea-level rises are roughly correlative with events from the Appala

and Great Basin (Harris et al., 1996) suggesting there were four to seven

similarity of these curves suggests the rise and falls in relative sea-level w

level fall in the Late Ordovician occurs during the Hirnantian indicating

Differences in the timing of maximum water depth in any individual lo

sediment supply.

(Chatfieldian–Hirnantian; 454–442 Ma) transgres-

sive to regressive sequence during the Late Ordovi-

cian containing 6–12 superimposed sea-level rises

and falls each 1–3 my apiece (Fig. 10). The Chat-

fieldian record is poorly resolved but it appears to

consist of two to five sea level rises and falls. The

Edenian rise and fall appears to be the longest

duration (f 3 Ma) but may locally record at least

two separate events. There are two long-term sea

level rise and fall events in both the Maysvillian

and Richmondian and a single long-term rise and fall

recorded in the Hirnantian. A very detailed record of

multiple high-frequency high-amplitude fluctuations

ea-level, coastal onlap and water depth curves from the periphery of

imates the local maximum flooding surface in each study area. The

ises and falls corresponding to the six depositional sequences. These

chian Basin (Holland and Patzkowsky, 1997; Pope and Read, 1998)

third-order rises and falls in sea-level in the Late Ordovician. The

ere likely produced by glacio-eustatic fluctuations. The largest sea-

the maximum extent of Late Ordovician icesheets during this time.

cation were likely produced by local variations in subsidence and


in the very latest Ordovician (Fig. 10) occurs on

Anticosti Island (Long, 1993).

The similarity in timing of these long-term events,

their duration, and distribution around Laurentia sug-

gests there is likely a long-term eustatic driver to these

relative sea-level fluctuations, but variations in subsi-

dence, sediment distribution or tectonics are likely

obscuring the true eustatic component of these curves.

For example, in the Appalachian Basin, the deepest

water facies in the Late Ordovician occurs during the

Chatfieldian (Diecchio and Broderson, 1994) or the

earliest Edenian (Holland and Patzkowsky, 1997;

Pope and Read, 1998). However, the deepest water

facies of the Montoya Group occur during the Mays-

villian (Fig. 10), whereas they occur in the Richmon-

dian in Great Basin rocks (Harris and Sheehan, 1996,

1997). Although, the absolute magnitude of these

long-term sea-level events is uncertain, it is clear

sea-level fell the farthest in the Hirnantian because

that lowstand led to widespread subaerial exposure of

Laurentia and elsewhere (cf. Brenchley et al., 1994),

probably during the maximum advance of Late Ordo-

vician icesheets.

The mechanism(s) creating these long-term relative

sea-level fluctuations around Laurentia during the

Late Ordovician is unclear. The duration of these

fluctuations, 1–3 my each, is generally below the

accepted record of long-term changes in seafloor

spreading rates that may cause eustatic variations

(Pitman, 1978). Similarly, these fluctuations are much

longer than the short-term Milankovitch climate forc-

ing mechanisms, precession, obliquity and eccentric-

ity, that have frequencies of approximately 21–23, 41,

and 100 or 400 ky, respectively. These fluctuations

may reflect long-term changes in ice volume, possibly

as a long-term (1.3 or 2.0 Ma) harmonic of eccentric-

ity (De Boer and Smith, 1994). However, if long-term

changes in ice volume are being recorded in the Late

Ordovician why is the record of short-term ice volume

changes, which should be represented by high-fre-

quency, high-amplitude depositional cycles be so

scarce? Moderate to high-amplitude, high-frequency

meter-scale cycles developed during the Chatfieldian

(Pope and Read, 1998) and the Hirnantian (Long,

1993), but unequivocable evidence for such cylicity in

the Cinncinnatian is not evident. Meter- and decame-

ter-scale depositional cycles are ubiquitous in the

Cincinnatian successions of the Appalachian Basin

(e.g. Holland, 1993; Jennette and Pryor, 1993; Pope

and Read, 1997b; Brett and Algeo, 2001). However,

determining the magnitude and duration of sea-level

fluctuations that produced this cyclicity is controver-

sial (cf. Jennette and Pryor, 1993; Holland et al., 1997

for contrasting views).

Alternatively, the nearly synchronous long-term

Late Ordovician fluctuations delineated here may be

recording nonlinear oscillations that are not directly

related to any eustatic forcing mechanisms (e.g. Gaf-

fin, 1992). More detailed sequence stratigraphic

records through Late Ordovician successions are nec-

essary to determine the mechanism(s) that produced

these long-term relative sea-level fluctuations. Re-

gardless of the mechanism(s), these long-term relative

sea level fluctuations they may provide a means for

regional and global correlations that are below the

resolution of biostratigraphy.

8. Conclusions

The Late Ordovician glacial event was unique

because it formed during a time of elevated atmo-

spheric CO2. Determining the duration of this glacial

episode is essential to determining how glaciation

began, how waxing and waning of the glacial ice-

sheets affected coeval sedimentation, and how and

why this glaciation ceased. The abundance of cherty

carbonate and phosphate in the Montoya Group of

New Mexico and Texas indicates these rocks formed

in a widespread upwelling zone. The duration of this

upwelling and correlation with other Late Ordovician

upwelling zones around the periphery of Laurentia

suggest that Gondwana glaciation may have been

occurring throughout this period. The regionally cor-

relative third-order depositional sequences (1–3 my

duration) in the Montoya Group refine a long-term

Late Ordovician relative sea-level curve.

Acknowledgements

This research supported by ACS-PRF #35837-G8.

Bob Myers, Range Geologist at White Sands Missile

Range provided invaluable help in accessing sections

in the San Andres range. Dan Hunter, Bryn Clark,

Luke LeMond, Steve Turpin and John Bengelsdorf all


provided invaluable field assistance. Steven Holland

and Carlton Brett provided insightful comments on a

previous version of this manuscript. Jessica Steffen

initiated many useful discussions about the Montoya

Group and especially the significance of its chert.

Kate Giles provided access to a digital camera, advice

on logistics and acted as a sounding board for many of

the ideas in this paper. This paper was approved for

public release by White Sands Missile Range;

distribution unlimited. OPSEC review completed on

August 19, 2002.

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