Incised-Valley Fills and Other Evidence of Sea-level Fluctuations Affecting Deposition of the Catskill Formation - JSR, Cotter & Driese, 1998

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JOURNAL OF SEDIMENTARY RESEARCH, V OL. 68, NO. 2, MARCH, 1998, P. 347–361Copyright 1998, SEPM (Society for Sedimentary Geology) 1073-130X/98/068-347/$03.00

INCISED-VALLEY FILLS AND OTHER EVIDENCE OF SEA-LEVEL FLUCTUATIONS AFFECTINGDEPOSITION OF THE CATSKILL FORMATION (UPPER DEVONIAN),

APPALACHIAN FORELAND BASIN, PENNSYLVANIA

EDWARD COTTER1 AND STEVEN G. DRIESE2

1 Department of Geology, Bucknell University, Lewisburg, Pennsylvania 17837, U.S.A.2 Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410, U.S.A.

ABSTRACT: An exceptionally complete and extensive new exposure of the Catskill Formation (Upper Devonian) in the Susquehanna Valleyof central Pennsylvania demonstrates that fluctuations of relative sealevel of at least two orders in the Milankovitch range affected its de-positional history well into the Famennian Stage. Seven incised-valley-fill (IVF) units, ranging in thickness from 10 to 33 m, are present upto about 600 m above the base of the Sherman Creek Member. Wherefully developed, they have the three-part vertical sequence recentlyproposed as the signature of IVFs: (1) above a basal scour surface,cross-stratified sandstone and conglomerate deposited in environmentsthat were initially high-intensity braided fluvial but that became in-creasingly affected by marine storm processes; (2) a medial unit of green–gray fissile mudrock containing symmetrical sandstone ripples

in flaser and linsen forms accumulated in the anoxic calm of the centralzone of an estuary; and (3) an upper unit of wave-rippled sandstoneand/or channelform sandstone representing progradational bayheadfilling of the estuary. Other marine influences on deposition of theSherman Creek Member are indicated by a diverse suite of brachio-pods and by wave-rippled sandstone. A rough estimate suggests thatthese marine incursions into the Catskill coastal–alluvial plain repre-sent fourth-order fluctuations of relative sea level.

Features of the Irish Valley Member confirm that deposition of fissilemudrock took place on the marine shelf under low-energy, anoxic con-ditions only intermittently interrupted by storms, and that thicker (2 m) sandstone units represent sharp-based shelf sand bars. Fifth-orderfluctuations of relative sea level produced numerous, repetitious shal-lowing-upward sequences in both paralic and shelf facies of the Irish

Valley.These interpretations provide the basis for sequence-stratigraphicanalysis of both members of the Catskill Formation as a unified de-positional system. In lowstand phases, Catskill streams erosionally in-cised their courses into the coastal–alluvial plain, bypassing coarsersediment to the shelf, where it accumulated as lowstand shelf fans. Inthe transgressive phase, incised valleys were drowned as transgressivesurfaces moved diachronously upvalley, changing braided fluvial sys-tems to storm-wave-influenced marginal-marine sand environments,followed by the establishment of anoxic, deeper estuarine conditions.On the shelf, transgressions restored muddy conditions of low-energyand anoxia. Highstand conditions resulted in filling of the incised valleyby aggradation in the estuary and progradation at the bayhead, grad-ually returning the area to pre-incision coastal–alluvial plain condi-tions.

INTRODUCTION

Subject of Study

The Catskill Formation (Upper Devonian) is one of a series of Paleozoicterrestrial stratigraphic units exposed in the Valley and Ridge Physiograph-ic Province in central Pennsylvania (Meckel 1970). From early in this cen-tury (Barrell 1913, 1914; Woodrow and Sevon 1985), the Catskill For-mation has been generally understood as a massive siliciclastic wedge thatprograded westward into the Appalachian Foreland Basin as a depositionalresponse to the Acadian Orogeny. In the outcrop of Upper Devonian strata

in the fold belt of central Pennsylvania, attempts have been made to inter-pret the details of this progradation, but the result has been what othershave called a ‘‘bewildering thicket of interpretations’’ (Slingerland andLoule 1988, p. 125). These investigations have tended to concentrate onindividual stratigraphic units in isolation from each other. An exceptionallyextensive and complete new exposure of the Catskill Formation in theSusquehanna Valley of central Pennsylvania (Figs. 1, 2) provides an op-portunity to analyze in a coherent manner, in the same stratigraphic section,Catskill facies that range from shallow shelf, through coastal and paralic,to fully terrestrial.

In central Pennsylvania, the Catskill Formation is subdivided, in ascend-ing order, into the Irish Valley, Sherman Creek, and Duncannon Members(Fig. 3) (Berg et al. 1983). Near the site of this investigation in the Sus-

quehanna Valley, the Catskill Formation is more than 2 km thick (Ayrton1963; Dennison 1982), with the Irish Valley Member making up about 650m and the Sherman Creek about 1000 m (Hoskins 1976).

It is impossible to specify precise ages for the Catskill Formation andits members in central Pennsylvania because of insufficient biostratigraphiccontrol and diachronous younging across the outcrop belt. Published strati-graphic correlation charts (Berg et al. 1983; Sevon and Woodrow 1985;Scheckler 1986; Woodrow et al. 1988; Warne and McGhee 1991) showthat the Catskill Formation extends from the middle of the Frasnian to thelatter part of the Famennian Stage (Fig. 3). The Irish Valley Member beginsin the upper Frasnian and continues approximately to the Frasnian–Famen-nian boundary, and the Sherman Creek Member extends from that bound-ary an undetermined distance into the Famennian.

General Late Devonian SettingCentral Pennsylvania in the Late Devonian was about 20 south of the

equator (Kent and Miller 1988; Van der Voo 1988), where the climate wastropically warm, seasons alternated between wet and dry (Driese and Mora1993), and stream discharge varied widely (Woodrow 1985). An alluvialplain sloped northwestward (present-day azimuth) from the eroding Aca-dian Mountains toward a low-energy, muddy coastal margin, and fromthere a shallow-marine ramp deepened toward the basin axis (Woodrow1985; Dennison 1985). The shoreline ran approximately northeast–south-west but was deeply embayed between riverine input centers (Slingerlandand Loule 1988). Rivers on the alluvial plain had braided patterns in easternPennsylvania but became single-channel and increasingly sinuous down-slope in central Pennsylvania (Glaeser 1974; Sevon 1985). As the alluvialplain prograded westward, the number of major rivers diminished as small-

er streams were captured by the expanding drainage basins of larger rivers(Sevon 1985; Boswell and Donaldson 1988). Bedload detritus in thoserivers was largely sand, with a relatively high percentage of metamorphicrock fragments (Meyer 1963; Rahmanian 1979), but other grains includedpedogenic carbonate, fish plates, phosphate nodules, and plant fragments.

Location of Study

Along the west side of U.S. Highway 11 and 15, about 5 km south of Selinsgrove, Pennsylvania (Fig. 1), highway widening has resulted in aseries of vertical cliffs up to 100 m high (Fig. 2), exposing a successionof about 1.6 km of strata of the Catskill clastic wedge with only minor



348 E. COTTER AND S.G. DRIESE

FIG. 1.—Location of Catskill Formationoutcrop on west side of US Highways 11/15south of Selinsgrove, Pennsylvania.

FIG. 2.—Nature of exposure of Catskill Formation at Selinsgrove location. Photoshows the part of the Sherman Creek Member from about 430 m to 510 m (see Fig.7). IVF Unit VI descends from upper right corner of photo, and IVF Unit VIIascends from lower left corner.

covered intervals. Strata strike approximately normal to the outcrop faceand dip almost homoclinally south at 30–45. The upper 450 m of the IrishValley Member are nearly completely exposed beginning about 3.2 kmsouth of the intersection of Highway 11 and 15 with Pennsylvania Route35. Conformably above the Irish Valley, the Sherman Creek Member is

exposed almost completely for about 630 stratigraphic meters and inter-mittently for another 100 m above that. Stratigraphically below the studiedunits, several hundred meters of Trimmers Rock Formation are continu-ously exposed.

Objectives

The broader objectives of this report include demonstrating from repe-titious facies sequences that fluctuations of relative sea level on at leasttwo scales have left their imprint on Upper Devonian strata in central Penn-sylvania. Marine incursions over the Catskill coastal–alluvial plain weremore numerous and extensive, and persisted later into the Famennian Stage,than heretofore realized. The report will also show that some of these sea-level fluctuations were of sufficient magnitude to erosionally incise deeper

valleys into the coastal depositional ramp, only to have them filled withestuarine deposits as sea level subsequently rose. A final broad objectiveis the illustration of a genetic link between the sedimentary histories of thecoastal–alluvial plain and the coeval shallow-marine shelf, a link estab-lished by the simultaneous response of both depositional settings to thefluctuations of relative sea level.

To accomplish these objectives, the report is organized in the followingway. The first part contains descriptions and interpretations of the individ-ual facies of both the Irish Valley Member and the Sherman Creek Memberof the Catskill Formation as they are exposed at the Selinsgrove outcrop.In this, emphasis is given to those facies central to new interpretations. Thenext part considers the associations and sequences of facies in order to



349 INCISED-VALLEY FILLS LINK DEVONIAN SHELF AND COASTAL-PLAIN HISTORIES

FIG. 3.—General stratigraphic correlation chart of Upper Devonian strata in central

Pennsylvania. Boundaries are diachronous in this proximal (right)–distal (left) tran-sect across the central Pennsylvania outcrop belt. Ages at the Selinsgrove outcropare approximately those shown near the left margin of the chart. From Berg et al.(1983) and Sevon and Woodrow (1985).

FIG. 4.—Stratigraphic column of exposed part of the Irish Valley Member of Catskill Formation at Selinsgrove outcrop. Interpreted shallowing-upward sequences areindicated by black wedges to right of columns. Letters A–G indicate units of light gray, cross-laminated sandstone. IV designates unit of inclined heterolithic stratification.Asterisks indicate the 19 shallowing-upward sequences included in calculation of mean cycle thickness.

propose depositional settings and histories for each member. Those separatedepositional histories are then integrated through a sequence-stratigraphicapproach that considers how the Catskill coastal margin and shelf respond-ed to cyclic fluctuations of relative sea level. The final section attempts to

estimate the periodicities of the two orders of apparently cyclic sea-levelfluctuations recorded in the Catskill succession in central Pennsylvania.

SEDIMENTARY FACIES

Irish Valley Member

The basic descriptive and interpretive elements of Irish Valley facies are

summarized in Table 1. Fuller descriptions and interpretations are providedbelow for two of these facies, for it is partly through the correct interpre-tation of these two facies that the origin of the Irish Valley Member canbe linked with that of the overlying Sherman Creek Member. The IrishValley also contains one unit of red, inclined heterolithic stratification(IHS); this facies is more characteristic of the overlying Sherman CreekMember and will be considered with that member.

Green–Gray Fissile Mudrock.—This predominant facies of the mem-ber occurs in units whose thickness ranges from more than 12 m near thebase of the exposure (Fig. 4, below 100 m) to less than 1 m toward thetop (above 350 m). Most of the mudrock is laminated in the form of mil-limeter-thick, sharp-based, flat streaks of coarser siltstone spaced throughthe finer mudrock. No cyclic patterns of thickening, thinning, or spacingwere detected. Some units of this facies (e.g., Fig. 4, 280–285 m, 320–325m) comprise laminated mudrock that contain mixtures of symmetrically

rippled, very fine-grained sandstone alternating with different proportionsof draped mudrock (linsen and flasers). Proportions of rippled sand anddraping mud vary nonsystematically. The parts with flasers and linsen gradeto and from mudrock flat-bedded laminites with no fining-upward sequenc-es formed by upward increases in the amount of mud.

Scattered through some units of this mudrock facies are very thin tomedium (1–30 cm thick) beds of very fine- to fine-grained sandstone (someshown as isolated single lines on the columns of Figure 4). Bases and topsof the sandstone beds are equally sharp, but while the bases are flat, the




FIG. 5.—Seven units of light gray, cross-laminated sandstone in Irish Valley Member. Letters A–G indicate positions in stratigraphic sequence (Fig. 4). See Figure 6 forphoto of Unit F.

top surfaces are commonly broadly wavy (hummocklike) and/or symmet-rically rippled. In several cases, the amplitude of the top-surface wavesexceeds the thickness of the bed, resulting in complete lateral pinching outof the sandstone. Internal structures comprise horizontal lamination, andlow-angle and hummocky cross stratification. In the lowest exposed partof the member (Fig. 4, 0–100 m), a number of thick beds have pronouncedbasal ball-and-pillow structures. Several sandstone beds contain abundantbioclasts of crinoids and brachiopods, either concentrated as basal lags orscattered sparsely throughout.

Interpretation.—This mudrock facies accumulated on a shallow-marineshelf below wave base under low-energy background conditions. The well-laminated fabric, with no body fossils or biogenic structures, indicates thatconditions were anoxic (Sageman et al. 1991). This is consistent with theshallow pycnocline proposed for the Catskill marine basin (Woodrow 1985;Ettensohn and Elam 1985; Baird et al. 1988). The generally oxygen-poornature of Late Devonian shelf waters (Berry et al. 1989; Savoy 1992) wasexacerbated by the abundance of plant matter and nutrients reaching theshelf from the adjacent land mass (Algeo et al. 1995).

Many other aspects of this facies were related to storms. Storm-entrainedsilt and sand were episodically flushed as density currents over the con-solidated mud (Hawley 1981a), and it is possible that some of the muditself was introduced by storms (Hawley 1981b). Similar interpretationshave been applied to other thinly laminated silty mudrock and to flasers

and linsen in mudrocks that range in age from Proterozoic to Cretaceous(de Raaf et al. 1977; Pedersen 1985; Schieber 1989, 1990; Davis and Byers1989 1993; Krassay 1994). Units of green–gray mudrock many metersthick that have only millimeter-thick distal siltstone storm interbeds weredeposited in relatively deeper conditions, below storm wave base (Aigner1985; Brenchley et al. 1993). Flaser and linsen structures were formed inmore proximal and/or shallower environments, where storm-introducedsand was reworked by waves related to the same storm (de Raaf et al.1977; Pedersen 1985; Davis and Byers 1989, 1993; Krassay 1994). Thetabular and lenticular sandstone beds in this facies were also generated bystorms, in a manner similar to that described by Aigner (1985), Brenchley(1985, 1989), Brenchley et al. (1993), Cotter (1990), and Myrow and Sou-

thard (1996). Many aspects of these beds are similar to those of storm-generated sandstones in shallow-marine, mudrock-dominated deposits inthe Upper Devonian Catskill clastic wedge of New York State (Craft andBridge 1987; Halperin and Bridge 1988; Bridge and Willis 1994).

For a variety of reasons, this facies cannot be interpreted in terms of atidal-flat origin. Fining-upward sequences typical of prograded tidal flats(Klein 1972; Weimer et al. 1982; Terwindt 1988) are not present, and thesilt-streaked laminites do not have the cyclic patterns of thickening andthinning of beds that are characteristic of tidal deposits (Nio and Yang

1991; Archer et al. 1995). Many mudrock units that contain flasers andlinsen also include flat-based tabular and lenticular sandstone beds, whichare characteristic shallow-marine storm deposits, not tidal-flat deposits.Thicknesses of some of the mudrock units containing flasers and linsenapproach ten meters or more (Fig. 4); if those units were formed in tidalflats, it is likely that the paleotidal range would have to have been inor-dinately large. That would then imply the existence of many tidal channels,which cannot be found in the green–gray mudrock facies. The laminiteshave the same characteristics in homogeneous units more than ten metersthick as they do where interbedded with flasers and linsen structures; adifferent interpretation of the origin of the silt-streaked mudrock is notneeded in those situations where flasers and linsen are also present.

Light Gray, Cross-Laminated Sandstone.—Thick units of light gray,cross-laminated, fine-grained sandstone are interbedded within the green–

gray fissile mudrock. Thicknesses of the eight units greater than 2 m thickaverage 4.2 m, and range from 2.5 m to 7.7 m (Figs. 4, 5). Basal contactsare sharp and most commonly flat, but some are broadly wavy, and one(Unit F) is deeply incised into the underlying mudrock (Figs. 5, 6). Manytop contacts are broad arches, but smaller-scale, hummock-like undulationsare also present (Fig. 5B, E, F), in some cases superimposed on the broaderarches. Most internal bed boundaries are also broadly undulatory, with thesame range of geometries as the shapes of the top surfaces of the units,resulting in the presence of hummocky and swaly forms and in lateralchanges of bed thickness. Convergence of a wavy top surface with a flatbasal surface has resulted in lateral termination of some beds (Fig. 5D, E).Most internal laminae are inclined moderately toward the northwest, but




FIG. 6.—Light gray, cross-laminated sandstone Unit F in Irish Valley Member(Figs. 4, 5). At right edge of photo unit is 7 m thick (from 343 to 350 m).

FIG. 7.—Stratigraphic column of exposed Sherman Creek Member at Selinsgrove outcrop. For explanation of symbols see Figure 4. Letters A–J designate units of inclined heterolithic stratification (Table 2). Roman numerals I–IX within cross-ruled braces designate marine-influenced sequences, most of which are incised-valley fills(IVFs).

some toward the southeast. Compound cross stratification is present in oneunit (Fig. 5B), and small-scale symmetrical ripples are in others. Fragments

of plants, with a great range of sizes and abundances, are common alongbed surfaces and cross laminae in all units; the oxidation of pyritized plantremains typically imparts a distinguishing rusty stain to outcrops of theseunits.

Both set thickness and the scale of undulatory structures diminish upwardin most units of this facies (Fig. 5B, C, E, F). Toward the bases of units,set thicknesses are greatest, reaching as much as 4 m. Broadly wavy bed-ding surfaces in lower parts give way to more pronounced arches andswales in upper parts. This trend is accompanied by an increase in theproportion of interbedded mudrock or coarse siltstone, along with plantdetritus. The combination of these has resulted in large, hummock-like

lenses of fine-grained sandstone draped or enveloped by mudrock, as atransitional upper part of some units (Fig. 5B, E). Swales between thesehummocks can contain very abundant fragments of plants. Very thin bedsof the overlying green–gray mudrock facies can be inclined at 10–15 ,parallel to the underlying hummock-like surface of the sandstone (Fig. 5D).

Interpretation.—This facies has many of the attributes of shelf sandbars (‘‘shoal complexes’’) (Johnson and Baldwin 1986; Winn 1991). Underthe influence of storm-generated processes combining vigorous seaward-directed currents and intense wave oscillation (Duke et al. 1991; Cheel andLeckie 1993), the bars migrated laterally across shelf mud. Many barsmigrated across a flat bottom, but in some cases they filled depressionsincised into the mud substrate (Figs. 6F, 7). Upper bar surfaces were broad-ly convex upward, and on some, late-stage storms superimposed smallerhummocks and swales. Lateral terminations occurred where relatively steepupper bar flanks descended to the level of the mud substrate. The greatabundance of plant fragments in all units demonstrates that detritus wastransported directly from a terrestrial source to the bars, but once there,shallow-marine processes molded that detritus into the bar forms. Throughthe period of bar development the intensity of shallow-marine processesdiminished from initially higher-power storm effects, through less-intenseand less-frequent storms, and finally a return to low-energy conditions andthe draping of shelf mud over the tops of the bars and the bar-top hum-mocks (Brenchley et al. 1993). Units of this facies resemble the ‘‘trans-gressive storm-dominated shelf successions’’ of Galloway and Hobday(1983, p. 159–160).

A tidal channel/shoal origin of these sandstone units cannot be supportedwhen the units are enveloped in shelf mud, typically have flat bases andbroadly arched tops, have structures characteristic of storm-wave produc-tion, and have no evidence of criteria for tidal processes, such as tidalbundles, mud-drape couplets, and sigmoidal bedding (Mutti et al. 1985;Kreisa and Moiola 1986; Terwindt 1988; Nio and Yang 1991; Dalrympleet al. 1992). A progradational shoreface origin is also not likely, for at their




TABLE 1.—Catskill facies characteristics and interpretations

Facies Lithology Sedimentary Characteristics Depositional Conditions

Irish Valley member facies:

Bioturbated quartz-rich sandstone

Fine-medium grained; quartz-domi-nant; some quartz gravel.

Sharp-based 5–30 cm beds; tops rippled & transitional. Internally churned by irregular large burrowers. Scatteredreworked brachiopods & crinoids; also fish plates, phosphate pellets, intraclasts.

Transgressive lags formed byshoreface truncation & condensa-tion (Kidwell 1989; Einsele1992).

Green-gray fissile

mudrock

Fine siltstone with mm-thick coars-

er siltstone laminae; plant debris;rare brachiopods & crinoids.

Units up to 12 m thick. Silt laminae flat to incipiently rippled. Ss. flasers & linsen abundant in some units. Some

very thin to medium, tabular to lenticular beds of fine ss. with flat, low-angle, & hummocky laminae; some haveabundant bioclasts.

Anoxic shallow-marine shelf below

wave base; coarser laminae andbeds from episodic storms (seetext).

Light-gray, cross-laminated sand-stone

Fine-grained, quartz-rich sandstone;plant fragments line laminae andconcentrate in silty lenses .

Units 2.5–7.7 m-thick; sharp bases either flat or erosionally scoured; tops broadly arched and/or undulatory. Bedsets0.2–0.4 m-thick and bounded by flat to broadly wavy surfaces; laminae inclined at low to medium angles (mostto NW, some to SE); some hummocky & swaley structures and symmetrical ripples. Upward decrease in bedthicknesses and scale structures, and upward increase in proportion of siltstone and plant detritus.

Shelf sand bars deposited undercombined-flow conditions; inten-sity of processes diminishedthrough time as shelf deepened(see text).

Red massive mudrockand rippled sand-stone

Siltstone and thin beds of very finesandstone .

Lower part transitional from green-gray fissile mudrock; contains 10–30 cm-thick zone of flasers and linsen. Majori-ty of unit is massive mudrock, with abundant mm-thick root traces, and some zones of blocky peds and slicken-sided surfaces. Contains 3–10 cm-thick very fine ss. beds that are symmetrically rippled. Desiccation cracks.

Exposed mudflats at the margin of a low-energy coast.

Sherman Creek member facies:

Very thick red sand-stone

Fine-grained lithic arenite; someunits have basal calcrete con-glomerate and/or abundant plantdetritus.

Mean thickness of 26 units is 2.85 m (std. dev. 0.78 m). Bases commonly erosionally scoured; bedding planes andlaminae mostly horizontal; some cross laminae directed to NW; uncommon lateral accretion surfaces. Tops of units transitional to mudrock-dominated red heterolithic facies.

Laterally migrated channels of sin-gle-channel, sinuous rivers

Mudrock-dominatedred heterolithic

Fine siltstone; thin interbeds of very fine-grained sandstone.

Units up to 50 m-thick; dominated by massive red mudrock that has abundant root traces, desiccation cracks, &caliche nodules. Thin sandstone interbeds typically have sharp contacts, are laterally continuous, and contain rip-ple lamination.

Overbank floodplains of alluvialsystem

Exceptional Sherman Creek facies:

Green-gray fissilemudrock

Fine siltstone; mm-thick coarsersiltstone laminae; some vf sand-stone flasers & linsen.

Nonsystematically spaced coarser siltstone laminae are flat, with incipient ripples; some zones have abundant flasers& linsen with no internal laminae; vf to fine sandstone in uncommon thin beds.

Low-energy, anoxic shallow-ma-rine/estuarine setting episodicallyand weakly influenced by storms(see text).

Symetrically rippledsandstone

Very fine and fine sandstone. Beds 3 to 5 cm-thick; top surfaces have symmetrical ripples with h 0.8 cm, l 10 cm; internal laminae notevident.

Wave agitated shallow coastal mar-gin.

Red inclined hetero-lithic sequences(IHS)

Fine-grained red sandstone anddarker red mudrock in alternatingbeds.

Units 1.7 to 7.9 m-thick (mean 3.4 m), with horizontal, parallel upper & lower unit boundaries; beds inclined 4 to20 (mean 10) degrees, with mostly straight (some sigmoidal) profiles; upper parts of some units have sparse roottraces and small carbonate nodules.

Marine-influenced distal reaches of fluvial channels (see text).

Red, matrix-rich sand-stone in channel-forms

Fine, matrix-rich sandstone; baseshave mudrock intraclasts and/orplant fragments

4–6 m-thick units with concave-upward symmetrical channelforms up to 3 m-thick; most internally structureless,but some have obscure cross lamination inclined about 20 to NW; No fining-upward trends.

Erosive scour followed by seaward-directed transport of land-derivedsediment and plants; likely bay-head delta.

Light gray, cross-laminated sand-stone

Fine, mod. sorted, quartz-rich sand-stone; subordinate conglomerateof caliche pellets, mudrock intra-clasts, fish plates, large plantfrags, and rare brachiopods.

Unit bases erosively scoured; tops flat or wavy to hummocklike. Undulatory to swaley surfaces of 30–80 cm-thickbeds bound medium-angle cross stratification; most inclinations to NW, some to SE. Plant detritus on laminae andconcentrated in swales, mostly in upper parts of units. Subordinate amounts of symmetrically rippled and flaseredsandstone.

Fluvially derived sediment accumu-lated in estuary channel that wasgradually transgressed and con-verted to shoreface conditions(see text).

bases most units do not have gradational transitions from more distal shelf facies, they do not coarsen and/or shallow-upward to paralic and terrestrialfacies, and they show not the characteristic shoreface vertical sequence(Reinson 1992). Rather, they indicate an upward decrease of storm-gen-erated intensity.

Summary of Irish Valley Depositional Conditions.—Accumulation of the Irish Valley Member took place on the shallow-marine shelf ramp andon the adjacent coastline. Most of the time the shelf sea was calm and well-stratified, leading to deposition of unburrowed mud, which now forms thedominant facies of the member, green–gray fissile mudrock. Also locatedon the ramp were shelf bar complexes that were deposited under combined-flow conditions, forming the light gray, cross-laminated sandstone facies.

Episodic storms not only molded the form of the sand bars, they alsoredistributed the sand to deeper water as beds, laminae, and small-scalelenses (linsen and flasers). The coastline consisted of low-energy, micro-tidal mudflats, and is recorded by the red massive mudrock and rippled

sandstone facies. Cyclic fluctuations of relative sea level in the Milanko-vitch range (see later section) produced the numerous alternations of green–gray fissile mudrock and red massive mudrock and rippled sandstone forwhich the Irish Valley Member is well known (Walker 1971; Walker andHarms 1971, 1975). Most boundaries between the red mudrock and theoverlying green–gray mudrock are marked by thin beds of the bioturbated

quartz-rich sandstone facies, which formed as transgressive lags.

Sherman Creek Member

Because the fluvial channel and floodplain facies of the Sherman CreekMember at the Selinsgrove outcrop are similar to those previously reportedfor this member, only a small amount of description and interpretation arepresented in Table 1 and in a later section on Sherman Creek facies se-quences. The five exceptional Sherman Creek facies are, however, pre-sented here in considerably more detail. Note that characteristics of the firsttwo of these exceptional facies are so similar to facies of the underlyingIrish Valley Member that the same facies names are used.

Light Gray Cross-Laminated Sandstone.—Seven prominent units con-taining relatively thick bedsets of light gray cross-laminated sandstone are

spaced through the Sherman Creek Member at Selinsgrove (Fig. 7). Basesof the cross-laminated bedset units in the Sherman Creek Member are sharpand in most cases scoured up to three meters into underlying strata of thered mudrock-dominated heterolithic facies (Fig. 7A, D). Many top bound-aries are also sharp, but some are more transitional to either symmetricallyrippled very fine-grained sandstone or to gray mudrock with flat silt streaksand linsen and flasers. Shapes of the top boundaries are either flat or wavyand hummock-like.

The light gray sandstone is fine-grained, moderately sorted, and quartz-rich. Most units also contain abundant conglomerates composed of calciumcarbonate (caliche) pellets, mudrock intraclasts (some red), fish plates, andnumerous large and small plant fragments. These conglomerates are thick-




FIG. 8.—Four incised-valley fill (IVF) sequences in Sherman Creek Member. Vertical bars demarcate extent of sequences; capital letters designate positions in stratigraphiccolumn (Fig. 7). Symbols are shown in Figure 4.

FIG. 9.—Wavy bedding and hummocky cross stratification at top of light graycross-laminated sandstone unit in IVF Sequence VII (Figs. 7, 10; 485–486 m). Flatlamination in green–gray fissile mudrock near top of photo demonstrates that un-dulations are not tectonic. Scale is 15 cm long.

est and most common just above the bases of the sandstone units, but theyare also found in shallow scours and lenses farther above the base (Fig.8C, D). Disarticulated brachiopods were found in loose blocks of the con-

glomerate at the foot of two different sandstone units (Sequences VI andVII, Fig. 8B, C). Plant fragments are strewn profusely on bed and laminasurfaces, and they are also concentrated in broad shallow swales betweensets of cross laminae. The oxidation of such pyritized plant concentrations

leads to rusty stains on the outcrop face. Interbedded within two of thelight gray sandstone units are thin to thick intervals of gray mudrock thatcontain flat silt streaks and/or linsen and flasers (Fig. 8C, 478 m; Fig. 8D,

572 m). Associated mudrocks also lack desiccation cracks, root traces, andin situ calcium carbonate nodules.The most characteristic sedimentary structure in the sandstone is low- to

medium-angle cross stratification, with angles of inclination that range fromabout 8 to 15. Bed surfaces have broad, undulatory forms that are bestdeveloped at the tops of some units (Fig. 8C, D), and cross laminae largelyhave the same shapes and inclinations as the surface forms (Figs. 8, 9).Almost all cross laminae are directed toward the northwest, but severalsoutheastward-inclined cross laminae occur in Sequence VI between 448and 450 m (Fig. 8B). Beds tend to become thinner upward, and bed surfacesbecome more undulatory; the upper two meters of the facies in SequenceVII (Fig. 8C, 484–486 m) contain pronounced wavy bedding that has thecharacteristics of hummocky cross stratification. Much of the abundantplant material in this facies is concentrated in concave-upward swales inthe upper parts of this facies.

Interpretation.—This facies formed as fluvially derived sediment wasmodified in an estuarine setting by shallow-marine processes, with an in-creasing influence of storm waves as the sand aggraded. Fuller justificationof this will come from interpretations of associated facies, as well as fromanalysis of Sherman Creek facies sequences (see below). A terrestrialsource of sediment is demonstrated by the abundance of caliche-pellet con-glomerates and profuse plant material (see Scheihing and Pfefferkorn1984). Many of the characteristics of the lower parts of this facies aresimilar to compound bar deposits of braided rivers (Roe 1987; Roe andHermansen 1993), although facies thicknesses far exceed those of ShermanCreek fluvial point-bar sequences (Sevon 1985; see Facies Sequences, be-




FIG. 10.—Inclined heterolithic stratification (IHS) Units B and C (Table 2) sepa-rated by channelform sandstone. As indicated in Fig 7, Unit B extends from 77.5to 82 m and has an apparent inclination of 5–6, whereas Unit C extends from 83to 86.5 m and has apparent inclination of 10–14. Staff is 1.5 m long.

TABLE 2.—Units of inclined heterolithic stratification, Catskill Formation, Selins-grove outcrop

Unit

StratigraphicPosition

(m)Thickness

(m) Apparent Dip Root Traces CaCO3

Proximity toIVF (m)

ABCD

EFGHIJIV

45.6–48.077.5–82.083.0–86.5

200.0–201.7

375.0–379.0463.0–464.9522.4–524.2564.0–568.2590.0–592.3607.0–614.9223.5–229.6

2.44.53.51.7

4.01.91.84.22.37.96.1

9–105–6

10–1414

4–612–1414–1610

428

10–14

FewFewFew; topSlight

FewRareFewTop onlyFew; top

NoneNoneNear topNear top

RareNoneFewTop onlyTiny; top

4182230

ContactContact

10Contact

630

Contact

low). Where the cross strata trend upward to more undulatory forms theybegin to resemble anisotropic hummocky cross stratification formed incombined oscillatory and unidirectional flows in which the unidirectionalcurrent component was dominant (Nottvedt and Kreisa 1987; Duke et al.1991; Cheel and Leckie 1993). To produce the more isotropic undulationsin the sandstone in Sequence VII (Figs. 12C, 14), the combined flow wasdominated by the oscillatory component. Criteria diagnostic of tidal pro-cesses (Terwindt 1988; Nio and Yang 1991) are absent.

Green–Gray Fissile Mudrock.—Units of green–gray fissile mudrock,with characteristics nearly identical to the facies of the same name in theIrish Valley Member, are present at several positions high above the baseof the Sherman Creek Member (Fig. 7, Units III, VII, IX; Fig. 8C, D).Lithologies include both silt-streaked laminites and mudrock containingvery fine-grained sandstone in the form of flaser and linsen structures. Some

parts become heterolithic where they contain thin interbeds of flat-lami-nated sandstone that have flat bases and top surfaces with well-developedinterference ripple patterns.

A diverse assemblage of disarticulated brachiopods is present in poorlysorted sandstone interbedded within the fissile mudrock in Sequence III(Fig. 7; 234 m). These brachiopods are intermixed with numerous largeand small fragments of plants. Although badly fragmented, taxa have beenidentified by G.R. McGhee (personal communication, 1996) as Tylothyrismesacostalis, Devonochonetes?, ‘‘Camarotoechia’’?/ Cupulorostrum?,Schizophoria?, and Cyrtospirifer.

Interpretation.—Characteristics of this facies, and their similarity withthe facies of the same name in the Irish Valley Member, demonstrate thatdeposition occurred in shallow-marine waters. Physical structures suggestthat conditions for the most part were calm and anoxic but intermittently

interrupted by the storm-generated introduction of coarser silt, rippled sand,and thin storm beds (see Brenchley 1985, 1989; Myrow and Southard1996). Association of brachiopods mixed with abundant plant fragmentsindicates a depositional setting transitional between marine waters (sourceof the brachiopods) and land drainage waters (source of the plants). Aswith the fissile mudrock in the Irish Valley Member, no evidence of tidal-flat conditions was found.

Red, Inclined Heterolithic Stratification.—Ten or more units of red,inclined heterolithic stratification (IHS) are spaced throughout the exposedSherman Creek Member, from near its base to more than 600 m above thebase (Figs. 7, 10; Table 2). Thicknesses range from 1.7 to 7.9 m, with amean thickness of 3.42 m. If the thickest unit is set aside, the average

thickness of the remaining nine units is 2.9 m, which is similar to the meanthickness (2.85 m) of the channel sandstone components of the 26 fluvialfining-upward sequences (Fig. 7; Table 1, very thick red sandstone; seeFacies Sequences, below). Upper and lower boundaries of most units areparallel and horizontal, with limited evidence of scour along the basal sur-face. Inclined strata extend through entire units, and their apparent dipsrange from 4 to more than 20 and average about 10. Most inclined beds

have relatively straight profiles, but in two units profiles are irregular withsigmoidal, benchlike segments. Lithologies consist of 3–10-cm-thick bedsof red fine-grained sandstone alternating with slightly thinner beds of darkerred mudrock. Most individual sandstone beds do not fine upward and dis-play very few primary structures. In three of the units, grain size diminishesupward by virtue of increase in the proportion of mudrock. Sparse roottraces increase in abundance upward in several units, and about half theunits contain a few very small carbonate nodules near the top. These IHSunits are most commonly overlain by redbeds of the horizontally bedded,mudrock-dominated red heterolithic facies, but many of them are in contactwith or close to units interpreted to be deposited partly under marine con-ditions (see Table 2, and Facies Sequences, below).

The Irish Valley Member contains a unit of IHS similar to those in theSherman Creek Member (Fig. 4, Unit IV, 224–230 m; Table 2). In thiscase, lithologies grade upward from red, fine-grained sandstone containing

wave-generated structures to red mudrock that has root traces and verysmall carbonate nodules. This unit overlies green–gray fissile mudrock thatcontains marine fossils, and it is capped by a thin transgressive lag asso-ciated with flasers and other indications of wave influence.

Interpretation.—An increasing number of reports appear to agree withthe proposition of Smith (1987, 1988) that many IHS formed in tidallyinfluenced settings (for example, see Shanley et al. 1992; Shanley andMcCabe 1993). On balance, attributes of the IHS units of the ShermanCreek and Irish Valley Members suggest that they indeed formed in mar-ginal-marine conditions associated with distal reaches of coastal–alluvialchannels. The IHS units are very different from Sherman Creek fluvialpoint bars, which are represented at this locality by the horizontally bedded,very thick red sandstone facies units at the bases of the 26 fining-upwardsequences (see Table 1 and Facies Sequences, below), yet their average

thickness is remarkably similar to the average thickness of the fluvial chan-nel sandstones. Additional support for this interpretation comes from theproximity of IHS units to interpreted marginal-marine deposits in the Sher-man Creek Member (Table 2), and from the similarity of the ShermanCreek units to the indisputable marginal-marine IHS unit in the middle of the Irish Valley Member.

This interpretation is muted, however, by the absence in the ShermanCreek IHS units of several of the diagnostic criteria for tidally influencedIHS summarized by Thomas et al. (1987). For example, most units lackobvious fining-upward trends in the unit as a whole as well as in individualsandstone beds; they have very few structures produced by tides or waves;bioturbation is scarce to absent; and they do not have marine or brackish




fossils as basal lags. Root traces and small carbonate nodules in upper partsof units indicate the passage of more time than a tidal cycle or even aspring–neap cycle.

Symmetrically Rippled Sandstone.—In some parts of the ShermanCreek Member symmetrically rippled sandstone dominates, to the near ex-clusion of mudrock. The most prominent unit occurs between 287 and 295m above the base of the member (Fig. 7, Unit IV); here the fine-grainedsandstone has ripples with spacing between 12 and 15 cm and heights of about 1 cm. No internal lamination is evident. Most rippled beds are es-sentially horizontal, but in some bedsets about 20 cm thick, slight incli-nations of about 10 are present. This unit of rippled sandstone overlies a15-cm-thick bioturbated conglomerate containing phosphate, skeletal, andintraclast grains, which in turn overlies red mudrock with root traces. Unitsof symmetrically rippled sandstone also occur at 382–384 m, 503–507 m,and 552–554 m above the base of the Sherman Creek (Fig. 7, Units V,VII, VIII). In each case, the ripples display foreset laminae that are inclinedtoward the southeast.

Interpretation.—Symmetrical ripples of this type appear to be charac-teristic of deposition in shoreface conditions (Howard and Reineck 1981;McCubbin 1982), and are best preserved where bioturbators are inhibited.Sandstones with similar ripples have been reported from shallow-marineCatskill strata in New York, about 200 km north of the Selinsgrove outcrop

(Craft and Bridge 1987; Halperin and Bridge 1988). The basal conglom-eratic bed is another transgressive lag on top of subaerial coastal plaindeposits.

Red, Matrix-Rich, Channelform Sandstone.—Several 4–6-m-thickunits of red, matrix-rich fine-grained sandstone contain broad, relativelysymmetrical, concave-upward surfaces that have as much as 3 m relief. Thesurfaces are in places lined with plant fragments and/or mudrock intraclasts.Neither the units nor the individual channelform segments exhibit fining-upward trends. Most channelform segments are internally structureless, butsome reveal obscure internal lamination inclined toward the northwest at15–20. Examples of these units are at 479–483 m and at 579–584 m (Fig.7).

Interpretation.—These features indicate that moderately high-energyerosive events were followed by seaward-directed transport of land-derivedsediment and plant fragments. The characteristics of this facies are dissim-

ilar to those of the coarser, channel parts of the fluvial fining-upward se-quences so common in the Sherman Creek Member (see Facies Sequences,below). Assisted by knowledge of its position in the exceptional ShermanCreek facies sequences, we consider that this facies was likely to have beendeposited in the upper reaches of an aggrading estuary, possibly as a bay-head delta.

Summary of Sherman Creek Depositional Conditions.—For quitesome time, the Sherman Creek Member has been identified as the productof deposition in sinuous fluvial channels and their adjacent floodplains(Barrell 1913; Sevon 1985). The Sherman Creek Member at the Selins-grove outcrop (Figs. 2, 7) does indeed consist largely of alternating unitsof the very thick red sandstone facies (fluvial channel deposits) and themudrock-dominated red heterolithic facies (overbank floodplain deposits).The Selinsgrove outcrop, however, also exhibits five marine-influenced fa-

cies that are exceptional for the Sherman Creek Member: symmetricallyrippled sandstone (wave-agitated coastal margin), red inclined heterolithicstratification (marine-influenced distal fluvial channels), light-gray, cross-laminated sandstone (transgressed distal estuary), green–gray fissile mud-rock (low-energy medial estuary), and red sandstone in channelforms (es-tuarine bayhead delta). The mutual association of these marine-influencedfacies in the Sherman Creek Member (see facies sequences, below) rein-forces the interpretation that the Catskill coastal–alluvial plain in centralPennsylvania was repeatedly affected by fluctuations of relative sea level.During regressive and lowstand phases of these fluctuations, Catskill riversincised their valleys into the plain. In the subsequent transgressive andhighstand phases, these incised valleys were filled by estuarine aggradation.

A fuller understanding of these interpretations will come from considera-tion of the associations and sequences of facies in the two members of theCatskill Formation at the Selinsgrove outcrop.

FACIES SEQUENCES

Irish Valley Facies Sequences

Description.—The Irish Valley Member has long been known to containrepetitive sequences consisting of alternating marine and nonmarine facies(Dyson 1963; Allen and Friend 1968; Walker 1971; Walker and Harms1971, 1975). At the Selinsgrove outcrop, the most common and easilyrecognizable Irish Valley facies sequence consists of the green–gray mud-rock facies (marine) alternating with the red massive mudrock and rippledsandstone facies (largely terrestrial), with transgressive lags of the biotur-bated quartz-rich sandstone facies resting above the red terrestrial facies.These sequences are broadly consistent with the general Irish Valley model(‘‘motif’’) presented by Walker (1971) and Walker and Harms (1971,1975). A less common variety of this sequence has abundant flasers andlinsen within the green–gray fissile marine mudrock. In still a third variety,thicker sandstone storm beds and/or units of light gray, cross-laminatedsandstone are contained within the green-gray marine mudrock. The twovariations of Irish Valley sequences found by Rahmanian (1979) (his ‘‘sil-

ty–muddy motifs’’ and ‘‘sandy–silty motifs’’) are equivalent to sequenceswithout and with light gray, cross-laminated sandstone units within thegreen–gray fissile mudrock.

The average thickness of the 34 Irish Valley facies sequences in thecompletely exposed part of the Irish Valley Member between 98 and 378m (Fig. 7) is 7.9 m, with a range of 1.6 to 27.4 m and a standard deviationof 6.5 m. Boundaries of sequences were defined at bases of the bioturbatedquartz-rich sandstone facies where it overlies the red massive mudrock andrippled sandstone facies, and at sharp bases of the light-gray, cross-lami-nated sandstone facies where it overlies the green–gray fissile mudrockfacies. For the subset of 19 facies sequences that shallow up to a peritidaldatum, the mean thickness is 6.0 m and the standard deviation 5.1 m.

These sequence thicknesses are comparable to those determined by otherinvestigators. Walker and Harms (1971, 1975) used other localities of theIrish Valley Member in central Pennsylvania to calculate that approxi-

mately 25 Irish Valley motifs had an average thickness of 25 m and a rangefrom 4 to 45 m. For more basinward, northwestern localities, Rahmanian(1979)) found between 15 and 20 Irish Valley cycles with thicknesses rang-ing from 1.5 to 28 m. Van Tassell (1987) reported that approximatelycontemporaneous Upper Devonian strata in nearby Virginia and West Vir-ginia have shallowing-upward cycles with an average thickness of 20–25m. Cycle number and thickness can, of course, vary from locality to lo-cality, but the greater number and lesser thickness of cycles at the Selins-grove locality are likely to be a consequence of not counting sequences inthe lowest 100 m of the exposed section (Fig. 4) and by the use of unitsof light gray, cross-laminated sandstone to define some cycle boundariesin shelf facies.

Interpretation.—These are shallowing-upward cycles caused by fre-quent and repeated fluctuations of relative sea level (Allen and Friend 1968;

Walker 1971; Walker and Harms 1971, 1975; Rahmanian 1979; Van Tas-sell 1987; Slingerland and Loule 1988). During rises of relative sea level,the sea transgressed over the vegetated and mud-dominated terrestrialcoastal margin, leaving thin transgressive lags (Kidwell 1989). As relativesea level stabilized and possibly lowered, sedimentation of marine mud andsand built the depositional ramp above sea level, changing the facies backto the red massive mudrock and rippled sandstone. In facies that formedseaward of the coastal margin, the fluctuations of relative sea level led toa different variety of facies sequence. There, the light gray, cross-laminatedsandstone facies accumulated through the formation and migration of shelf sand bars (shoal complexes) at times of relatively lower sea level. Withrises of relative sea level, these bars dropped below levels of storm wave




FIG. 11.—Schematic longitudinal proximal–distal profile of interpreted incised-valley fillbased on IVF sequences in the Sherman CreekMember. Vertical bars and roman numeralsindicate interpreted positions of particular IVFsequences (Figs. 7, 8).

and current activity, returning the shelf to the placid calm and anoxia inwhich the green–gray fissile mudrock facies was deposited.

Sherman Creek Facies Sequences

Typical Sherman Creek Fining-Upward Sequences.—The exposedSherman Creek Member exhibits 26 fining-upward sequences in whichunits of very thick red sandstone facies are overlain by thicker units of mudrock-dominated red heterolithic facies (Fig. 7; Table 1). These faciessequences are closely similar to those at other sites of the Sherman CreekMember in central Pennsylvania (Sevon 1985), as well as in approximatelycontemporaneous Upper Devonian alluvial strata in adjacent parts of NewYork (Bridge and Gordon 1985; Gordon and Bridge 1987; Willis andBridge 1988). For the 26 fining-upward sequences exposed, thicknesses of the coarser sandstone facies averages 2.85 m (standard deviation 0.78).

Interpretation.—Magnitude of the Sherman Creek rivers can be esti-mated from the dimensions of the channel sandstone units in the Selins-grove outcrop. By adding a 10% compaction correction (Ethridge andSchumm 1978) to the relatively uniform mean thickness of 2.85 m, theriver depth at bankfull stage can be estimated to have been little more than

3.0 m. This is consistent with other estimates of bankfull depth of ShermanCreek rivers in central Pennsylvania (Rahmanian 1979; Sevon 1985) andfor the approximately contemporaneous Devonian rivers about 200 kmalong the alluvial plain from Selinsgrove in south-central New York (Gor-don and Bridge 1987). For the Duncannon Member, which overlies theSherman Creek Member in central Pennsylvania, Diemer (1992) used adetailed paleohydraulic reconstruction to demonstrate that there were twosize classes of rivers: larger trunk rivers that had mean depths of 3.5 m,and smaller tributaries that were about 1.8 m deep. The Sherman CreekMember rivers at the Selinsgrove outcrop appear to represent only one sizeclass, which was intermediate between the two sizes of Duncannon rivers.

Thicknesses of the red, mudrock-dominated floodplain facies are morevariable. These thicknesses are controlled by a number of complex factors(Mackey and Bridge 1995), among which are the stability of the river

meanderbelt on the coastal–alluvial plain and the relationship between thechannel and its base level (Reid and Frostick 1994). Rises of base levelassociated with fluctuations of relative sea level (see below) can also leadto long-term vertical accretion of floodplain deposits (Reid and Frostick1994). In the floodplain facies units themselves, mudrock with more nu-merous and thicker sandstone interbeds formed closer to a river meander-belt, whereas thick mudrock units without sandstone interbeds record pe-riods when the meanderbelt was more distant (Reid and Frostick 1994).

Sequences of Exceptional Facies.—Most of the exceptional ShermanCreek facies (Table 1) are associated with each other in a series of ninefacies sequences (numbered I–IX in Figure 7). Thicknesses of these se-quences range from 8 to 33 m. Each sequence is sandwiched within mud-

rock-dominated red heterolithic facies (alluvial floodplain) that contain roottraces, desiccation cracks, and carbonate nodules (Figs. 7, 8). Sequencebases are scoured into the underlying red mudrock; above each sequence,strata conformably return to red mudrock. Many of the sequences, more-over, are closely associated with units of red inclined heterolithic sequences

(Fig. 7; Table 2).A general composite of these sequences has three parts: a basal unit of

light gray, cross-laminated sandstone and conglomerate rests above an ero-sional scour, the medial part is dominated by green–gray fissile mudrockthat in places contains flaser and linsen structures, and the upper unit con-sists largely of fine-grained sandstone with symmetrical ripples. ThickerSherman Creek sequences (Sequences VII and IX, Figs. 7, 8C, D) containall three parts of the general model. The medial mudrock part is poorlydeveloped in Sequence VI and altogether absent in Sequence V (Fig. 8A).Some sequences, such as V (Fig. 8A) are compound and contain repetitionsof one or more of the three basic parts.

Interpretation.—The sequences of exceptional facies in the ShermanCreek Member represent the fillings of valleys incised into the Catskillcoastal–alluvial plain. Many of their aspects fit the characteristics of estu-

arine deposits (Reineck and Singh 1980; Clifton 1982; Dalrymple et al.1992; Dalrymple et al. 1994a). Useful summaries of incised-valley fill(IVF) characteristics have been presented by Dalrymple et al. (1992), Dal-rymple et al. (1994b), Zaitlin et al. (1994), and Allen and Posamentier(1994).

Even with the considerable variability that exists among IVFs, a generalmodel has been developed that presents the facies architecture of IVFs interms of three longitudinal (proximal–distal) valley segments and a three-part vertical organization (Rahmani 1988; Dalrymple et al. 1994b; Zaitlinet al. 1994). In this general model, facies that accumulated in distal reachesof the incised valley were dominated by marine processes, facies in themiddle segment had a mixed marine and fluvial history, and facies in theinnermost part of the valley reflect only fluvial processes. Over much of the length of the incised valley, the bottom of the IVF consists of alluvialsediments; these are transgressively overlain by muds and sands of thecentral estuary and nearshore; and at the top, deposits return to alluvial.

The sequences of exceptional Sherman Creek facies have characteristicsthat fit diagnostic criteria for IVFs (Dalrymple et al. 1992; Dalrymple etal. 1994a). Sequence thicknesses are significantly greater than that of coevalSherman Creek fluvial channels, and gravel lags rest on erosionally scouredbasal surfaces. In vertical facies order, basal parts are dominated by alluvialdeposits, medial parts contain muddy estuarine and marine deposits, andupper parts are primarily sandy, prograded bayhead deposits. Specific Sher-man Creek vertical sequences (Figs. 7, 8) can be interpreted within thegeneral model as variants along the proximal–distal spectrum of IVF char-acteristics (Fig. 11). The thicker sequences with well-developed three-part




FIG. 12.—Idealized paleoenvironmentalreconstruction depicting integrated depositionalsystem interpreted for Irish Valley and ShermanCreek Members at the Selinsgrove outcrop.Letters next to lines suggest possible locationsfor development of light gray, cross-laminatedunits in Irish Valley Member (Figs. 4, 5, 6).Roman numerals next to lines suggest possible

locations for development of IVF sequences inSherman Creek Member (Figs. 7, 8).

vertical organization (Sequences VII and IX; Fig. 8C, D) formed in moredistal, outer reaches of an incised valley. There, following erosional scour,IVF deposition began as high-intensity braided fluvial systems transportedsand and gravel seaward. Marine transgression subsequently superimposedan increasing degree of wave influence and some landward transport onthis coarser sediment before the estuary deepened to quiet, anoxic, muddyconditions. The estuary gradually shallowed as sediment prograded at thebay head, exposing deposits to wave winnowing before accretion couldbuild the alluvial plain back up above sea level. Thinner sequences withreduced or missing medial fissile mudrock (Sequences V and VI; Fig. 8A,

B) accumulated in more proximal, inner reaches. The compound fills dem-onstrated by Sequences V and VII (Fig. 8A,C) indicate that sea-level fluc-tuations of more than one order were involved in their formation (Zaitlinet al. 1994) (see below).

INTEGRATED DEPOSITIONAL SYSTEM AND SEA LEVEL

Overview

The characteristics analyzed above make it possible to integrate the sep-arate depositional histories of the Irish Valley and Sherman Creek Mem-bers. Important components in this linking are the reinterpretation of twomarine facies (green–gray fissile mudrock with flasers and linsen, and lightgray cross-laminated sandstone) in the Irish Valley Member, and recogni-tion of those same marine-influenced facies within the Sherman Creek

Member. The spatial and genetic relationships among the facies of the twomembers are illustrated in Figure 12. The integrated depositional historycan be understood through application of sequence-stratigraphic concepts(Posamentier et al. 1988; Posamentier and Vail 1988; Van Wagoner et al.1988; Weimer and Posamentier 1993), wherein stratal packages can beviewed in terms of deposition during specific stages in the cycle of sea-level change (Posamentier and James 1993). The characteristically low-energy coast and shelf recorded by the Irish Valley facies were contem-poraneous with the more proximal Sherman Creek coastal–alluvial plain.Both members contain the light gray cross-laminated sandstone because attimes of low relative sea level, sand, gravel, and plant detritus were trans-ported down incised valleys and spread onto the shelf depositional ramp,

where they were modified by storms to form shelf sand bars. When relativesea level subsequently rose, the incised valleys were drowned; in the re-sulting estuaries, marine-influenced facies, including the green–gray fissilemudrock, were deposited as parts of the Sherman Creek Member.

Sequence Stratigraphic Development

Lowstand Phase.—Streams incised their courses into the Catskill coast-al–alluvial plain as relative sea level fell toward lowstand phases. The val-ley floor was degraded, and relatively coarse sediment was transportedbasinward, in the manner described by Wood et al. (1993a, 1993b) and

Zaitlin et al. (1994). Some of this derived sand, fish plates, and plant de-tritus bypassed proximal shelf areas to accumulate as sharp-based lowstandshelf fans (Posamentier and Vail 1988; Van Wagoner et al. 1988; Posa-mentier et al. 1992), probably under combined-flow conditions (Brenchleyet al. (1993). Both the erosional bases of the incised-valley fills and thebases of the shelf sand bar units are Type 1 sequence boundaries (Posa-mentier and Vail 1988) related to lowstand erosion (Bhattacharya 1993).

Transgressive Phase.—When valley degradation ceased with rises inrelative sea level, fluvially derived coarser sediment aggraded in the valleysand no longer reached the shelf. Some initial coarse valley deposits, suchas the log-rich silty sandstones at the base of IVF Sequence VI (Fig. 8B),resemble basal parts of modern estuary fills (Clifton 1982). Further rise of sea level led to upvalley migration of a transgressive surface, although inmost cases it appears that the microtidal and low-wave-energy conditions

of the Catskill Sea hindered the development of ravinement surfaces. InIVF Sequence VII, this transgression is recorded in the light-gray cross-laminated facies by the vertical change from unidirectional downvalleytransport to storm-wave-influenced deposition (Fig. 8C, 481 m). Water pro-gressively deepened, and distal parts of incised valleys became sites of lower-energy deposition of mud that had the same characteristics as shelf mud (green–gray fissile mudrock facies) (IVF Sequences VII and IX; Figs.8C, D). Maximum flooding surfaces (Van Wagoner et al. 1988; Zaitlin etal. 1994) are located within the central parts of these mudrock intervals.

These same rises of relative sea level affected deposition on the shelf depositional ramp. Shelf sand bars gradually became too deep for storm-wave reworking, and conditions returned to placid, anoxic low energy.




Initial flooding surfaces, equivalent to those near the bases of the incised-valley fills, can be placed in the Irish Valley Member where light graycross-laminated sandstone is overlain by green–gray fissile mudrock (seeBrenchley et al. 1993). At the shoreline, marine flooding produced thetransgressive lags of the bioturbated, quartz-rich sandstone facies in theIrish Valley Member.

Highstand Phase.—When the rise of relative sea level slowed and high-stand conditions were attained, the incised valleys became progressivelyfilled by sediment from both the fluvial system at the bayhead and themarginal marine system at the baymouth (Dalrymple et al. 1992; Zaitlin etal. 1994). In IVF sequences of the Sherman Creek Member, this final stageis recorded by a coarsening upward from marine mudrock to wave-influ-enced fine sandstone that exhibits sourceward (SE) transport. The filling of the estuaries returned the coastal–alluvial plain to pre-incision conditions.Catskill deposition then returned to the aggradation of sediment in mean-dering river channels and on the contemporaneous floodplains. The low-energy margin of the coastal–alluvial plain prograded seaward, completingthe shallowing-upward sequences seen in the Irish Valley Member (Fig.2). Very little sand accumulated at the shoreline and on the shelf, for muchof the coarser fluvial sediment was trapped in the infilling estuaries.

CYCLICITY OF SEA-LEVEL FLUCTUATIONS

Irish Valley Cyclicity

Apparently cyclic repetition of 34 Irish Valley shallowing-upward faciessequences is well displayed in the 280 m between 98 and 378 m of theSelinsgrove section (Fig. 2). Mean cycle thickness is 7.9 m (see abovesection on facies sequences). It appears that the same cycle thickness char-acterizes all of the exposed Irish Valley Member, although some other partsof the Irish Valley are not as continuously exposed. If that is correct, theentire 650 m of the Irish Valley would contain a total of 82 cycles.

The periodicity of these cycles can be roughly estimated by dividing thenumber of cycles by the total elapsed time. The elapsed time during whichthe Irish Valley Member accumulated is poorly constrained by availablebiostratigraphic evidence (Slingerland and Loule 1988; Warne and McGhee1991). Van Tassell (1987, 1994a, 1994b) estimated that the Irish Valley

formed over a period of 2.0 to 2.5 my, and Craft and Bridge (1987) usedan elapsed time of 2 to 3 my for the partly contemporaneous West FallsGroup in nearby New York. If the duration of the Irish Valley Member atSelinsgrove was approximately 2.5 my, the recurrence interval of the 82cycles would have been approximately 30 ky. This uncertain, rough esti-mate serves only to demonstrate that Irish Valley cyclicity is in the rangeof high-frequency, fifth-order parasequences (Mitchum and Van Wagoner1991).

An estimate of the magnitude of the fluctuation of relative sea level cancome only from cycles that shallow up to a peritidal datum (Osleger andRead 1991). The Irish Valley Member contains a subset of 19 cycles thatshallow up to coastal-margin paleosols with root traces (Fig. 4). Those 19cycles have a mean thickness of 6.0 m, with a standard deviation of 5.1m. Since it is not possible to make the necessary corrections for compac-

tion, sediment accumulation rate, and tectonic subsidence (Kendall andLerche 1988), it is only possible to conclude that relative sea level fluc-tuated with a range between five and ten meters.

Several arguments support the contention that these Irish Valley cyclesare of eustatic origin. Much of the early and middle Paleozoic was char-acterized by small-scale eustatic fluctuations, with magnitudes less than 10m (Wright 1992; Elrick 1995). In the Appalachian Foreland Basin, throughmuch of its Paleozoic development, many cycles of this same scale devel-oped in a great variety of siliciclastic and carbonate depositional systems(Dennison 1989; Cotter 1990; Dennison and Ettensohn 1994). Worldwide,on a number of formerly separated paleocontinents, Frasnian strata recordcycles of relative sea level with approximately the same range of thick-

nesses and calculated periodicities as these Irish Valley cycles (Cotter1991).

Sherman Creek Cyclicity

The large number of IVF sequences and other marine units spacedthrough the exposed member (Fig. 7) provide evidence for repeated fluc-tuation of relative sea level during deposition of the Sherman Creek Mem-ber. The magnitude and frequency of these fluctuations can be estimatedonly very crudely. It is not even clear that they were cyclic. With along-strike variability of coasts (Martinsen and Helland-Hansen 1995), it is un-likely that every sea-level fall is represented in this single outcrop by anincised-valley fill. Interfluve sites between the incised valleys are morelikely to record lowstand events as hiatuses and well-developed soils (Zait-lin et al. 1994), but such distinctive surfaces have not been identified inthe mudrock-dominated floodplain facies of the Sherman Creek Member.Thus, it is only possible to say that a minimum of nine marine incursionsare recorded in the Sherman Creek at the Selinsgrove outcrop.

Published correlation diagrams (Berg et al. 1983; Sevon and Woodrow1985; Woodrow et al. 1988; Warne and McGhee 1991) propose that de-position of the Sherman Creek and Duncannon Members occupied nearlythe entire Famennian Stage, which was approximately 4 my in duration

(Harland et al. 1990, p. 41). Here in the Susquehanna Valley, the ShermanCreek Member represents about 75% of the total thickness of these twomembers, so an assumption can be made that Sherman Creek depositionoccupied slightly less than 3 my. The exposed Sherman Creek has only630 m of the total 1000 m of the total member thickness, so it would haveaccumulated in approximately 1.8 my. With nine marine incursions in 1.8my, the mean recurrence interval would have been about 200 ky. Becauseit is likely that there were more fluctuations of relative sea level than arepresently recognized at the Selinsgrove outcrop, this crude estimate is onlya maximum. Presenting some credence to that very rough estimate is theproposal by Filer (1994) that cyclic fluctuations of relative sea level of equivalent magnitude and frequency have been reported from other, con-temporaneous Frasnian and early Famennian strata in the Appalachian Ba-sin. He documented cycles whose thicknesses typically are 20–30 m ormore, and whose estimated recurrence intervals are unequal and from 100

to 300 ky. Filer identified these as fourth-order cycles on the basis of thetaxonomy of Mitchum and Van Wagoner (1991).

Thicknesses of the IVF sequences provide a means to estimate the mag-nitude of the fluctuations of relative sea level. The generation of IVF Se-quence VII would have required a fall and subsequent rise of at least 33m (Figs. 7, 8). Smaller thicknesses of the other IVF sequences imply rel-ative sea-level fluctuations of lesser magnitude but in all cases greater than10 m. The thicknesses of the IVF sequences also indicate that the ShermanCreek cycles had an allogenic origin, for incision of tens of meters intothe coastal–alluvial plain is highly unlikely to have resulted from the au-togenic switching of outlet positions of three-meter-deep rivers.

The Sherman Creek IVFs contain evidence that two orders of sea-levelfluctuations operated simultaneously. The compound fills of several IVFsreveal that depositional conditions varied episodically, with smaller-scale

repetitious depth changes in which periods of valley scour and fluvial sed-iment aggradation alternated with deposition of marine mudrock (Sequenc-es V, VII, IX, Fig. 8A, C, D). Further evidence is the presence of inclinedheterolithic sequences throughout the Sherman Creek Member (Table 2;Fig. 7). Thus, within the fourth-order cycles represented by the ShermanCreek incised-valley fills are smaller-scale cycles that are closer in mag-nitude and periodicity to the fifth-order cycles interpreted from the IrishValley Member (see above). This simultaneous operation of sea-level fluc-tuations of more than one scale is consistent with the conclusions of VanTassell (1987, 1994a, 1994b) that a hierarchical series of sea-level cyclesaffected deposition of Frasnian and Famennian strata in this part of theAppalachian Basin.




DISCUSSION

The scenario described above is similar to interpretations presented forthe fillings of valleys incised into other ancient coastal margins. Many suchincised-valley fills were reviewed by Dalrymple et al. (1994a, 1994b) andZaitlin et al. (1994). Particularly apposite analogues for the Catskill valleyfills reported here are those described from Cretaceous strata in the forelandbasin east of the Rocky Mountains in North America, with many of the

investigations having taken place in the Canadian Province of Alberta. Adecade ago, Rahmani (1988) recognized paleoestuarine fills within valleysincised into Upper Cretaceous strata in Alberta, and he pointed out that acharacteristic signature for such fills was a tripartite (coarse–fine–coarse)vertical stratigraphic organization similar to that found in Catskill IVF Se-quences VII and IX (Fig. 8). In their summary, Dalrymple et al. (1992, p.1141) pointed out that such a vertical organization appears to be the generalpattern of organization of the deposits of an estuary filled as sea level risesfrom lowstand.

A number of the Cretaceous incised-valley fills reported from Albertaoffer instructive parallels with aspects of the Catskill examples describedin this report. Bhattacharya (1993) also showed that some of the fillings of Upper Cretaceous incised channels in the Dunvegan Formation of Albertaalso had the characteristic three-part vertical organization consisting of a

coarser, river-dominated basal unit, a mudrock-dominated, estuarine centralunit, and a sandstone top unit. Working with the upper Albian VikingFormation in Alberta, Pattison and Walker (1994) found incised valleysthat were filled as part of the transgressive systems tract. It was their judg-ment that the cutting and filling of these valleys was a response to fourth-order fluctuations of relative sea level, with a range of some 30–50 m,equivalent to the values interpreted for the Catskill Formation in this study.Walker (1995) made a similar estimate of 30 m for the maximum depthof an incised valley recorded in the Upper Cretaceous Cardium Formationin the subsurface of Alberta. Earlier, Leckie and Singh (1991) had alsofound valleys approximately 20 m deep incised into the Cretaceous shore-line of Alberta; in this case, these valley fills were associated with units of inclined heterolithic stratification. Finally, Wood (1996) found the fillingsof incised valleys in two different units of the Cretaceous Mannville inter-val in southern Alberta. He interpreted those that were filled predominantly

with muddy inner to middle estuarine deposits to have formed at moredistal positions than those that had coarser, predominantly fluvial fills. Theapproximately thirty meter thickness and the vertical architecture of someof the Mannville valley fill sequences (Wood, 1996, fig. 12) resemble veryclosely the characteristics of some of the vertical sequences of Catskillincised-valley fills (see Fig. 8, IVF Sequence VII).

CONCLUSIONS

The Catskill Formation (Upper Devonian) in the Susquehanna Valley of Pennsylvania records deposition on a coastal–alluvial plain and adjacentshallow-marine shelf under the influence of fluctuations of relative sea levelof at least two orders. In the Irish Valley Member, repeated alternation of shelf and paralic facies resulted from fifth-order fluctuations of relative sea

level with a magnitude of between five and ten meters and a periodicity of something like 30 ky.The Sherman Creek Member has heretofore been interpreted as entirely

nonmarine, but spaced through most of the exposed 630 m are exceptionalfacies that are very similar to marine facies in the underlying Irish ValleyMember. These exceptional Sherman Creek facies are organized in three-part vertical sequences that are now recognized as the fills of incised valleys(IVFs). This illustrates that fluctuations of relative sea level led to erosionalincision of the coastal–alluvial plain and the subsequent filling of thoseincisions with estuarine deposits. A minimum of seven IVFs in the exposedSherman Creek Member suggests that well into Famennian time, the Cat-skill depositional ramp was affected by fourth-order fluctuations of relative

sea level with magnitudes largely between 10 and 30 m and periodicitiesbetween 100 and 200 ky.

Sequence-stratigraphic analysis demonstrates the integration of the de-positional histories of the Irish Valley and Sherman Creek Members. Dur-ing lowstand phases, high-energy streams incised valleys into the ShermanCreek coastal–alluvial plain. This resulted in the transport of coarser sed-iment to the Irish Valley shallow shelf, where it accumulated as lowstandshelf fans. Transgressive-phase conditions returned low-energy, anoxic,muddy conditions both to the Irish Valley shelf and into the estuaries inthe Sherman Creek valleys. With returns to highstand conditions, sand-dominant sediment filled the Sherman Creek incised valleys and muddyand sandy sediment prograded the Irish Valley coastline.

ACKNOWLEDGMENTS

George McGhee of Rutgers University graciously evaluated and identified bra-chiopods found within the Sherman Creek Member. This paper was improved byreview of a preliminary version by Donald L. Woodrow and Martin R. Gibling.Journal reviewers B.A. Zaitlin and J.A. Diemer provided careful and constructivecriticism. Several student projects, including those of Jonathan Terry and AndrewLeier of Bucknell University, Laura Howe of Alfred University, and Daniel Capelleof the University of Tennessee contributed to knowledge incorporated in this report.This research was supported in part by a Faculty Summer Research Fellowship fromthe Petroleum Research Fund to the first author (Grant PRF 25678-AC8-SF94) and

by National Science Foundation Grant EAR-9418183 (to S.G.D.).

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Received 8 April 1997; accepted 16 October 1997.

Documents

Incised-Valley Fills and Other Evidence of Sea-level Fluctuations Affecting Deposition of the Catskill Formation - JSR, Cotter & Driese, 1998