Part II. Stratigraphy and Sedimentary Tectonics

CHAPTER 6 SILURIAN AND TRANSITION TO DEVONIAN

CHRIS1DPHER D. LAUGHREY INTRODUCTION Bureau of Topographic and Geologic Survey Department of Conservation and Natural Strata of Silurian age (405-430 Ma) constitute Resources only about 10 percent of the relative volume of Paleo­ 400 Waterfront Drive zoic sedimentary rocks in the central Appalachian Pittsburgh, PA 15222 basin (Colton, 1970), but they impart a dominant and characteristic physiographic form to the Appalachian Mountain section of the Ridge and Valley province of central Pennsylvania (Figure 6-1). Good exposures of Silurian rocks occur in the eastern part of the state and in the Appalachian Mountain section (Figure 6-2). Silurian rocks in the subsurface are very im­ portant petroleum reservoirs in the western part of the state (see Chapter 38B). Knowledge 'of Silurian lithologies in the subsurface of western Pennsylvania is based on oil- and gas-well cores, drill-cutting sam­ ples, and geophysical data. The demonstrable thickness of Silurian rocks in the state ranges from about 1,200 feet in northwestern Pennsylvania to almost 4,000 feet in eastern Penn­ sylvania (Figure 6-3). The Silurian System consists of two distinct depositional sequences: (1) a mainly Lower Silurian clastic sequence that extends through­ out much of the state as a thin succession of sand­ stones, conglomerates, and subordinate mudrocks and carbonates; and (2) the Upper Silurian portion of a Silurian-Devonian carbonate sequence that consists of a moderately thick succession of limestones and dolomites and other minor, but significant, lithologies (Colton, 1970).

LOWER LLANDOVERIAN In Early Silurian time, large volumes of clastic sediment were transported westward into central and northwestern Pennsylvania from eastern high­ lands raised during the Late Ordovician to Early Silu­ rian Taconic orogeny. Three lithostratigraphic units in three geographic regions are recognized. These are the Shawangunk Formation in eastern Pennsylvania; the Tuscarora Formation in central Pennsylvania; and the Medina Group in northwestern Pennsylvania. The

91 _ - CY' VI r- _,l;;."( Z --

~

SCALE

10 20 30 40 50 MI i i i , o i ',,'i i i ',,'i i i 20 40 60 80 KM !=' !;: c £

~

41·

/- YORK 40· -_/--, - ADAMS\

~ \ ) / I . __ { DEL.,. ,~~.~~~~~~ ------~-----'---~------~-~--..----;------~ 77°770 MD. 760 I

Figure 6-2. Distribution of Silurian rocks at the surface (solid color) and in the subsurface Oine pattern) in Pennsylvania (from Pennsylvania Geological Survey, 1990). CHAPTER 6-SILURIAN AND TRANSITION TO DEVONIAN 93

Figure 6-3. Isopach map show­ ing the thickness of Silurian strata across Pennsylvania. Contour interval is 500 feet.

Shawangunk Fonnation is divided into members and this area, the thickness of the Tuscarora ranges be­ facies (Epstein and Epstein, 1972), the Tuscarora tween 492 and 656 feet. It thins to the northwest to Formation into facies (Cotter, 1982, 1983a), and the a minimum thickness of about 200 feet. The Tusca­ Medina Group Group into formations and informal facies rora Formation lies conformably on the Upper Ordo­ (Piotrowski, 1981; Pees, 1983a; Laughrey, 1984). vician (Ashgillian) Juniata Fonnation. Conformably above it is the RQse Hill Shale, which is appointed a Eastern Pennsylvania­Pennsylvania­ late Llandoverian age. The Early Silurian age assign­ Shawangunk Formation ment for the Tuscarora is based on the ages of the juxtaposed formations because there are no datable In eastern Pennsylvania, the Shawangunk For­ fossils in the Tuscarora (Berry and Boucot, 1970). mation comprises all of the Llandoverian and much of Rocks of the Tuscarora Formation consist of quartz­ the Wenlockian Series (Berg, McInerney, and others, ose, sublithic, and argillaceous sandstones and shales 1986). The maximum thickness of the Shawangunk (Folk, 1960; Cotter, 1982, 1983a; Wescott, 1982). occurs at the Delaware Water Gap, where the forma­ Body fossils are conspicuously absent from the Tus­ tion is 2,100 feet thick. The upper contact with the carora Formation, but a small number of trace fos­ Bloomsburg Formation is irregular and transitional, sils, including Arthrophycus, Skolithos, and Monocra­ whereas the contact with the underlying Martinsburg terion, occur in varying abundance among different Formation is an angular unconformity (Epstein and lithofacies (Cotter, 1982, 1983a). Cotter (1982, 1983a) Epstein, 1972). Lithologies in the Shawangunk include interpreted the eastern part of the Tuscarora Forma­ coarse conglomerate, quartzose sandstone, and shale. tion as mostly fluvial in origin and the western part The Shawangunk is sparsely fossiliferous. Fragments as mostly marine (Figure 6-5). of lingulid brachiopods, eurypterid remains, and rare Dipleurozoa (a class of jeUyfishlikejellyfishlike fossils in the phy­ Western Pennsylvania-Medina Group lum Cnidaria) occur in the Lizard Creek Member. Trace fossils reported from the Shawangunk include The Medina Group, which has no outcrops in the feeding burrow Arthrophycus alleghaniense and Pennsylvania, is a sequence of quartzose, sublithic, the vertical burrow Skolithos. Epstein and Epstein and subarkosic sandstones, shales, and minor carbon­ (1972) interpreted the sedimentary characteristics of ates. It ranges in thickness from 200 feet to less than the Shawangunk as indicative of deposition in fluvial 140 feet. Workers in northwestern Pennsylvania gen­ and paralic environments (Figure 6-4). erallyeraUy report the Medina as disconformably overly­ ing the red shales of the Ordovician Queenston For­ mation and conformably underlying the carbonates Central Pennsylvania­ and shales of the Clinton Group. Body fossils are Thscarora Formation relatively common in the upper and middle parts of The Tuscarora Formation is distributed over a the Medina Group but are notably absent in the low­ large area of central Pennsylvania. Through most of ermost Whirlpool Sandstone (Fisher, 1954). Trace 94 C.C. D.LAUGHREY

Northwest

SHALLOW

SOURCE ROCKS

• 0 0

EXPLANATION

'".-.-- ~'-'-' Alluvlated Tidal flats Barrier zone coa stal plain

Figure 6-4. Block diagram showing sedimentary environments and major lithofacies in the Shawangunk Formation (from Epstein and Epstein, 1972, Figure 22). fossils are abundant in the Medina Group and have tics deposited on a coastal plain, and the Medina rocks proved quite useful for interpreting the origin of the of western Permsylvania are deltaic with offshore facies rocks (Laughrey, 1984; Pemberton and Frey, 1984; represented farther to the west (Figure 6-8). Accord­ Pemberton, 1987). Numerous workers have attributed ing to Smosna and Patchen (1978, p. 2,310), how­ the origin of the Medina Group to depOsition in a va­ ever, "Superimposed on this east-west gradation ... riety of fluvial, deltaic, paralic, and marine sedimen­ is a dual origin recognized for the Tuscarora., .. " tary environments (Figure 6-6) (Kelley, 1966; Kelley This twofold origin is apparent in all of the deposi­ and McGlade, 1969; Martini, 1971; Piotrowski, 1981; tional schematics presented in Figures 6-4, 6-5, and Laughrey, 1984; Duke and Fawcett, 1987), 6-6. Facies analyses of the Shawangunk Formation, Tuscarora Formation, and Medina Group all reveal Stratigraphic Correlations and Cyclicity the imprint of fluvial or paralic and marine environ­ ments, although fluvial and transitional facies domi­ The correlation of lower Llandoverian lithostrati­ nate the Shawangunk rocks of eastern Pennsylvania, graphic units across western Pennsylvania is shown and marine facies dominate the Tuscarora and Medina in Figure 6-7. A consensus developed among earlier rocks of central and western Pennsylvania. workers that the extensive and unbroken continuity Cotter's (1983a) investigation of the Tuscarora of Shawangunk, Tuscarora, and Medina lithologies Formation and work by Duke (1987b) employed con­ across Pennsylvania and adjacent states represented temporary techniques of "sequence stratigraphy," that a large, but simple, onshore-offshore complex from is, "the attempt to analyze stratigraphic successions in east to west (Yea~el, 1962; Knight, 1969; Martini, terms of genetically related packages of strata" (Num­ 1971; Smosna and Patchen, 1978; Piotrowski, 1981). medal, 1987, p. iii). This approach promises to pro­ This interpretation, most concisely presented by Yeakel vide a dynamic and realistic understanding of Lower (1962) and Smosna and Patchen (1978), implies that Silurian stratigraphy in Pennsylvania. Duke (1987b) the Shawangunk and Tuscarora rocks are alluvial clas- has suggested subdivisions of the Medina Group based CHAPTER 6-SILURIAN AND TRANSITION TO DEVONIAN 95

,...~. SHELF SAND·WAVE COMPLEXES '0 • 0 Q 0 A~:: ~~ rot 0°0°0 of, 0 0 0 0 C) 1.°.0000 JOoo o () 0 o /.0 0. 0 /0°0 00 0 0 0 0 0 00 0 -6 0 0 00 /1).0 0 00 A": 0 a 0 0 0 0 0 COASTAL ALLUVIAL PLAIN /0 0 .00 0 0 0 0 0 0 0 0 iO 0 ° () 0 0 0 0 ,,;!f5' 00000000 ,,'" ~. 0 0 0 0 0 0 0 () 0 0 () /OHarnsburg 0 0 0 0 0 0 /t" ()oooo·ooo 00 ° .-.<"1)0. 0 0 0 0 0 0 0 0 0 ° 000 ...... 0 °0. 0 .00 cOO 0 00 /1)°00.0.°.0 0 0 ./11 0 0 0 0 " " 00 0 0 0 0 " 0 " 0 0 0 0 0 0 /° 0 0 0 ALLUVIAL-FAN COMPLEX 0 0 ~ /00 0 0 0 a • 0 /! 00 a 0 0 0 0 0 a 0 0 • 0 0 • 0 • 0 0 • 0 0 a ° /.0 • '00 a 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 a 0 0 0000 '00 0 0 0. 000 SCALE 1 • 00 0 00 0 0 .0 0 ,.00°0 ·0 00 0 o. 0 0 o 10 20MI /0 0 0 0 0 0 0 0 0 0 0 a 0 0 0 11----"1-I.' t 0 0 -,.---.,.1.-,.---.,..1. Do"Ooo 00°°0° o 10 20 30KM /0 0 o. 0 0 0 • 0 • 0 ,,000 0 00 00 00

Figure 6-5. Cotter's (1983a, Figure 13, p. 42) interpretation of depositional environments in which the Tuscarora Formation originated during sea-level rise in earliest Llandoverian time. Shoreline and shelf facies migrated southeastward over the coastal alluvial-plain sediments with rising sea level.

Castanea Member Upper Tuscarora Formation

Figure 6-6. Generalized reconstruction of depositional environments during deposition of the Medina Group in northwestern Penn­ sylvania (from Laughrey, 1984, Figure 21). 96 C.C. D.D. LAUGHREY

A

VERTICAL SCALE M FT 150 40 100 20 50 \ A' o1 0

Figure 6-7. Correlation of lower Llandoverian lithostratigraphic units across western Pennsylvania (modified from Heyman, 1977, and Piotrowski, 1981). from Correlations are based on subsurface geophysical (gamma-ray) logs.

upon the correlation of erosional unconfonnities bound­ depositional cycles in the Medina could be attributed ing lithologically heterogeneous depositional cycles. to a number of possible causes, including tectonic The author has attempted to recognize these cycles processes, regional and/or global climatic variations, in whole-diameter cores of the Medina Group from global sea-level fluctuations, or oscillatory process­ northwestern Pennsylvania and tentatively correlate response systems. them with those suggested by Duke (1987a) (Figure 6-9). Duke (1987a, p. 18) proposed that "The un­ UPPER LL conformity-bounded cyclic ANDOVERlAN­ depositional sequences W recognized in the Medina are essentially identical to WER WENLOCKIAN I depositional cycles recognized ... by Cotter (1983a) The lithostratigraphic architecture of the Lower ! in the time-equivalent Tuscarora Formation .... " Silurian clastic sequence reveals a relatively uniform Cotter (1983a) ascribed the origin of depositional cy­ record of sedimentation across Pennsylvania and adja­ cles in 'I the Tuscarora Fonnation to the interplay of cent parts of the central Appalachian basin. A signifi­ eustatic sea-level fluctuations with tectonic and depo­ cantly different sedimentary pattern emerged during sitional events. Duke and Fawcett (1987) stated that Middle Silurian time, however. This new pattern, de­ fined by an elongate basin oriented northeast-south­ west (Figure 6-10), prevailed into the Devonian Pe­ riod (Dennison and Head, 1975). The central, or axial, part of the basin remained deeper than the margins through differential subsidence and limited sediment Tuscarora alluvIal· plain faciesfacIes influx (Smosna and Patchen, 1978). Low-energy mud facies were deposited along the basin axis, whereas the southeast and northwest basin margins were re­ gions of higher energy deposition.

Silurian rocks absent /') ,------".--_.1 Eastern Pennsylvania­ Shawangunk Formation Figure 6-8. Interpretation of the Shawangunk, Tus­ carora, and Medina lithologies across Pennsylvania as In eastern Pennsylvania, the Middle Silurian is a simple onshore-offshore complex (modified from represented by the upper part of the Shawangunk Piotrowski, 1981, Figure 3). Formation. Coarse-grained fluvial sands continued CHAPTER 6-SILURIAN AND TRANSITION TO DEVONIAN 97

THICK- FACIES CYCLES DEPTH GAMMA-RAY NESS (terminology of Duke (after Duke and (feet) LOG (meters) and Brusse, 1987) Brusse, 1987) 4600- 60

DEPTH OF0 F WATER 0 ~ , EXPLANATION

G ...... p \- Phosphatic lag 'f5 " "" zZ F Dolostone

4650- Bioclastic carbonates ----- Shale

Shale and sandstone

4700 - 30 ...... Sf( Sandstone and shale

Planar-bedded or parallel-laminated 20 sandstone :¢.$.:¢.%. Trough-crossbed­ 4750 - ~-- ded sandstone

10 A+B Channel-fill sandstone

Rippled sandstone

4800 - o

Figure 6-9. AUostratigraphic cycles in the Medina Group recognized in the Creacraft No.1 weD core from Crawford County. The gamma-ray log of the entire cored interval is shown on the left. The facies symbols used in the graphic core description are those of Duke and Brusse (1987). Letters and arrows on the right represent unconformity-bounded cyclic depositional sequences and relative sea-level changes proposed by Duke and Brusse (1987). The recognition of Duke and Brusse's cycles in the core supports their idea of cyclicity in these rocks and suggests that correlation of these Medina cycles with those recognized by Cotter (1983a) in the Tuscarora may be possible. I' jl i!it ~_~ "I) ~~'-J~~'-I 98 C.C. D.LAUGHREY

. ! . 90°9~O~0 __~ __~~~~ __~ __~ __~~ __r- __~-' Figure 6-10. Map showing axis of elongate basin and "form lines" that indicate subsidence and sedi­ 44° mentary accumulation patterns during times of low detrital input during the middle and late Paleozoic (from Dennison, 1982, Figure 3). This basin archi­ tecture nrst appeared during Middle Silurian time.

400 ILL. Ii I IND. The Keefer Formation conformably overlies J the Rose Hill Formation (Figure 6-11). The Keefer ~ ,.., I r -" contains quartz-cemented fossiliferous quartzose I ,J sandstone, hematitic oolitic sandstone, and minor y,.-...... r ./v ,-.J-' KY. mudrock. Sandstones of the Keefer Formation are very fine to coarse grained, silty, locally conglomer­ atic, crossbedded, and ripple bedded. Fossils include 36° crinoid stems, brachiopods, and mollusc shells. ----,,------The trace fossil Skolithos is locally abundant. I The Mifflintown Formation is composed of I I interbedded shallow marine mudrocks and lime­ ,I I stones. It conformably overlies the Keefer Forma­ tion and underlies the Bloomsburg Formation (Faill SCALE 32° ! I 100 202000 MIMI and Wells, 1974). r-ri',-'o-+--r-lo-r " i " " ' Cotter and Inners (1986) suggested that the I 100 200 300 KM I Rose Hill, Keefer, and Mifflintown Formations of central Pennsylvania accumulated on a submarine ramp that deepened from the proximal basin mar- to be deposited by streams bordering the southeast­ gin on the southeast to the basin axis at the approxi­ ern margin of the newly developed basin (Smosna mate position of the modem Allegheny Front (Figure and Patchen, 1978). These deposits are identified as 6-12). ,I the Tammany Member, which is the ',',,Uppermost mem- Cotter (1988,(1988, pp.. 242) recognized two "hierarchi­ ber of the Shawangunk Formation (tash and others, cally superimposed cycles of sea-level fluctuations" in 1984). The underlying Lizard Creek Member of the the medial Silurian succession of central Pennsylva­ Shawangunk, however, has much in common with nia. His lithostratigraphic interpretations (Figures basin-margin facies developed to the the west (see com­ 6-11 and 6-12) suggest that five large-scale cycles of ments by Lash and others, 1984, p.p. 83). transgression and regression, with a mean recurrence interval of about 2.5 million years, occurred during Central Pennsylvania-Rose Hill, Keefer, Middle Silurian time, and that these cycles governed the development of the observed lithostratigraphic and Mifflintown Formations framework at the level of formations and members The Middle Silurian succession in central Penn­ (Cotter, 1988, p. 242-245). Smaller scale cycles (3.3 sylvania is represented by the Rose Hill, Keefer, and to 9.8 feet in thickness) of sea-level fluctuations, pos­ Mifflintown Formations (Figure 6-11). The Rose Hill sibly related to Milankovitch climate cycles, are super­ Formation consists mostly of olive shale and also imposed on the larger scale transgressive-regressive contains minor purplish shale and thin beds of hema­ cycles and are correlative between different contem­ titic sandstone (Cabin Hill and Center Members). poraneous facies (Cotter, 1988, p. 244-245). The terms "lower shaly member," "middle shaly member," and "upper shaly member" are used in­ formally to designate the intervals of Rose Hill mud­ Western Pennsylvania-Clinton Group rocks below, between, and above the hematitic sand­ The Clinton Group constitutes Middle Silurian stones. Thin beds of fossiliferous limestone also occur strata in western Pennsylvania. The dominant unit within the Rose Hill near the top of the upper shaly in these strata is the Rochester Shale (Figure 6-11), member. which consists of a variably fossiliferous, gray mud- CHAPTER 6-6- SILURIAN AND TRANSITION TO DEVONIAN 99

DISTAL PROXIMAL NORTHWEST SOUTHEAST

Vernon Formation

Lockport Dolomite I

)ioherm Mifflintown Formation (-_ ...... ', .. ~-tJ.;.. :('--:.: I . ... --"',.-... ~ ) --' - - ' Rochester Shale " " " "" """" .. """"."""""""":."."." .... "--!'":""" ." : ::> :": :-: h= Irondequoit Dolomite

I IHi~tus I I I ... ~l. Williamson e Rochester Shale and Sodus ~ Shales. Rose Hill Formation undivided "": Cabin HUI Membe~.:: ::": ~ j ~W~O~IC~01~1 ~Li~m~eS~1o~ne~~~~~==-- ~~==~=c== ~ Sodus Shale Lower shaly member

....::.::: .:.~.. ':. .:::;=:~: ,C::;i :. /":'., ~.~~::. :c.' :.--.: "-:: :'::--i:--:·:·:;'~.· :: :' c.;, :.: ...... : :. '=: :~;:. Medina Group

APPROXIMATE HORIZONTAL SCALE o 100 MI Ir------~'-ri-----L--~--~----~I~I o 150 KM Figure 6-11. Cross section showing Middle Silurian stratigraphic units and lithofacies in central and western Pennsylvania. The cross section is oriented normal to the Appalachian basin axis in central and western Pennsylvania (from unpublished illustration by Edward Cotter, Bucknell University). stone and numerous interbedded carbonates (Brett, which are interbedded with the Rochester Shale (Nel­ 1983). The carbonate interbeds are interpreted as son and Coogan, 1984). The lower units of the Clin­ evidence for episodic, storm-dominated sedimenta­ ton Group lose their identity to the southeast and tion on a gentle southeast-sloping ramp (Brett, 1983). merge into the distinctive Rose Hill-Keefer sequence Bioherms occur near the top of the Rochester interval (Heyman, 1977; Figure 6-11). toward the basin axis (Cuffey and others, 1985; Fig­ ure 6-13)" The Rochester is recognized as a member of the Mifflintown Formation in central Pennsylvania UPPER WENLOCKIAN-LOWER (Berg, McInerney, and others, 1986). The lower part of the Clinton Group consists of LUDLOVIAN interbedded carbonate rocks (Irondequoit, Wolcott, Eastern Pennsylvania­ and Reynales Dolomites, Figure 6-11), mudrocks Bloomsburg Formation (Williamson, Sodus, and Ro.chester Shales), and minor sandstone (Thorold Sandstone). These lithologies are The Bloomsburg Formation of Pennsylvania is equivalent to the Brassfield Limestone, which crops composed of grayish-red claystone, siltstone, and out in southern Ohio. The Brassfield splits westward clayey, very fine grained to coarse-grained sandstone in the subsurface into several carbonate rock units, with small amounts of conglomerate (Hoskins, 1961). ~100 C.D.LAUGHREY

Figure 6-12. Interpretive model of Middle Silurian paleoenvironments in central Pennsylvania on the southeastern side of N the Appalachian basin (from Cotter and t Inners, 1986, Figure 14).

Foraminifera, bryozoans, brachiopods, mol­ luscs, ostracodes, crinoids, and fish scales have been described from the Bloomsburg Formation, but most are rare except for brachiopods and ostracodes. A generalized stratigraphic section of the Bloomsburg Fonnation and its correla­ tive units is shown in Figure 6-14. The upper and lower contacts of the Blooms­ • CLINTON " burg are conformable. The entire Blooms­ TIDAL FLATS burg Formation represents a time-trans- gressive unit. Deposition of the Blooms­ MARGINAL·SHELF SAND FACIES burg sediments began in late Wenlockian time and continued well into Ludlovian time. The top of the Bloomsburg Formation is in­ creasingly young toward the east (Hoskins, 1961; PALEOENVIRONMENTS -----r---.------, Berry and Boucot, 1970). Off .reef' Sand I Traced to the west and southwest from the type mud apron area in central Pennsylvania, the red beds of the w Bloomsburg Formation thin and are separated into two red-bed units by the marine limestones and shales of the upper member of the Mifflintown Formation

Monotrypa (Figure 6-14). The upper red unit continues to carry the name "Bloomsburg" in the west; the lower red BryolIIIlII AIg- ftrepaat ...al 1b1u1-GTllll1 unit is included in the Mifflintown Formation and is ORGANISMS __.l-. __ ...I- ______.l-._-J

~ ____~A~ ____~ called the Rabble Run Member. The upper portion

COLONY FORMS: BflANCHIiG HEADUKE of the Bloomsburg Formation contains a persistent sandy unit named the Moyer Ridge Member that is II'IIIy IimIllOIII I Muaive Tlin-bedded Muaive \Str;:'o., end slllil calcarenite branch coquinl reef rock ru~ L LITHOTYPES __.&...- _____--'- ___..Io....-_....J traceable over much of central Pennsylvania. The Bloomsburg Formation probably represents part of a large volume of deltaic sediments that were deposited over an area from Virginia into New York McKenzie Member and possibly into northern Michigan (Hoskins, 1961; c: c Smosna and Patchen, 1978). The sediments are thought 3: c .s '';::; 1-----" to have been deposited in waters sufficiently saline to .5 E allow a brackish-water fauna to exist. A few local de­ -:t::c... posits of nonred quartzose sandstone are interpreted -:::::ELI- Rochester Member as bar or beach deposits that were reworked suffi­ ciently to remove the clay and coloring matter.

Figure 6-13. Paleoecological interpretation (top) and Central Pennsylvania­ stratigraphic position (bottom) of a Middle Silurian reef McKenzie Formation near Lock Haven, Clinton County. Patch reefs such as this one developed on muddy Appalachian sea bottoms The McKenzie Formation of central Pennsyl­ (from Cuffey and others, 1985, Figures 2 and 3). vania underlies and and laterally interfingers with the CHAPTER 6-SILURIAN AND TRANSITION TO DEVONIAN 101

M FT 250 50 SCALE 125 20 40 MI 0 0 20 40 60KM 20 lewistown ? Bedford 8

Vertical exaggeration approximately x420 EXPLANATION

lB==~ Wills Creek Formation • Bloomsburg Formation t :1 Mifflintown Formation

~.;.:.X::'p:.);:::"nin Williamsport Member of Moyer Ridge Member of ~ Rabble Run Member of :,1'~.:::·., :: : Wills Creek Formation Bloomsburg Formation ~ Mifflintown Formation

8 Measured section described DRedbedS ? by Hoskins (1961)

Figure 6-14. Generalized northeast-southwest stratigraphic section of the Bloomsburg Formation and its cor­ relative units, approximately parallel to the Appalachian fold belt (modified from Hoskins, 1961, Figure 6).

Bloomsburg Formation. In much of central Pennsyl­ and Smosna, 1975). It ranges in thickness from ap­ vania, the McKenzie is designated as the upper mem­ proximately 200 to 300 feet. The upper and lower ber of the Mifflintown Formation; the McKenzie is boundaries are conformable. Fossils are sparse and given formational status in areas where other units of include brachiopods, ostracodes, gastropods, and the Mifflintown Formation caIUlot be distinguished favositid corals (Cotter, 1983b). Coral/stromatoporoid (see Berg, Mcinerney, and others, 1986).1986), bioherms in the McKenzie have been described by The McKenzie Formation is composed of dark­ Patchen and Smosna (1975) and Inners (1984). Ripple olive to gray marine shales containing thin interbed­ marks, megaripple bedding, and trace fossils are com­ ded marine limestone and minor siltstone (Patchen mon (Figure 6-15). The overall depositional envi-

Figure 6-15. Outcrop of limestone of the McKenzie Member of the Mifflintown Formation at Castanea, Clinton County (from Nickelsen and Cotter, 1983, Figure VlII-2A, p. 189). Note the megaripples and ripples on the surface of the limestone and the interbedded shale. The latter is the dominant lithology. Photograph by R. Sacks. 102 C.D.LAUGHREY

rorunent was open marine to intertidal. These envi­ DEPTH (feet) rorunents fluctuated with sea level during late Wen­ 3000 lockian time. I&i f'""

Western Pennsylvania-Lockport Dolomite ~ The McKenzie Formation grades laterally into SALINA ...... the Lockport Dolomite in northwestern Pennsylvania. GROUP 3100 Although it consists predominantly of dolomite, the Lockport contains some limestone (Rhinehart, 1979; Laughrey, 1987). It has an average thickness of 200 ~ feet in the subsurface of northwestern Pennsylvania. The Lockport is divided into five members at its out­ crop in western New York (Zenger, 1965; Crowley, 3200 1973). Such formational subdivisions cannot be re­ solved by the parastratigraphic format utilized to recognize operational units in the subsurface of north­ LOCKPORT western Pennsylvania (Forgotson, 1957; Heyman, DOLOMITE 1977). Only the basal DeCew Member can be rec­ 3300 ognized using the gamma-ray format (Figure 6-16). Cores and well cuttings of the Lockport Dolo­ mite appear brownish gray and buff to dark gray. The

carbonate rocks are finely to moderately crystalline ~ and contain intraclasts, ooids, peloids, and numer­ DeCEW 1.-0 DOLDOLOOMMITEITE ous fossils, including stromatoporoids, corals, echino­ 3400 ... ~ denns, bryozoans, molluscs, and brachiopods (Figure • 6-17). The Lockport Dolomite is a shallowing-upward ..roo. sequence (sensu James, 1979). Microfacies analysis CLINTON (Wilson, 1975) suggests that most of 'the Lockport GROUP I"~ was deposited subtidally in reef and interreef envi­ ronments (Zenger, 1965; Crowley, 1973; Rhine­ ,. 3500 I, hart, 1979; Shukla and Friedman, 1983; Laughrey, 1987). In the upper part of the Lockport, however, Figure 6-16. Gamma-ray geophysical log showing the some evidence exists for intertidal and supratidal stratigraphic position and geophysical character of the deposition. This includes associations of ooids, stro­ Lockport Dolomite and the DeCew Member in the subsurface of northwestern Pennsylvania. The log is matolites, and rip-up clasts, evaporite minerals, and from the Mary Mills No.1 well, Erie County. sabkha-type dolomite (Shukla and Friedman, 1983; Laughrey, 1987). conformably overlies the Bloomsburg Formation. The Bossardville Limestone consists of very thin bedded to UPPER LDLDDLOWAN-LOWER laminated, argillaceous limestones and lesser amounts of calcareous shale. Ostracodes are the dominant fos­ PRIDOLIAN sils. The Bossardville averages 100 feet in thickness. Northeastern Pennsylvania-Poxono Island It and the subjacent Poxono Island Formation were Formation and Bossardville Limestone' deposited in supratidal, intertidal, and subtidal marine environments (Epstein and others, 1974). Epstein and The Poxono Island Formation and Bossardville Epstein (1967) stated that there was a deepening of the Limestone lack fossils that can be used for age assign­ basin with time, causing a shift from supratidal depo­ ment but are considered Pridolian in age on the basis sition to subtidal deposition in a restricted lagoon. of stratigraphic position (Berry and Boucot, 1970). The Poxono Island Formation consists of lami­ Central Pennsylvania-Wills Creek and nated to fmely bedded, mud-cracked, lenticular dolo­ Tonoloway Formations mite, limestone, and calcareous shale. Fossils include brachiopod fragments and ostracodes. The Poxono The Wills Creek and Tonoloway Formations rep­ Island is approximately 140 to 200 feet thick and resent the upper Ludlovian and lower Pridolian global CHAPTER 6-SILURIAN AND TRANSITION TO DEVONIAN 103

N s

60

50

140 en [B 30 z ::.: u X 20 I-

10

10 20 30 40 50 60 70 80 90 HORIZONTAL SCALE (feet) EXPLANATION B t:;;::?t Stromatoporoid-cap facies 1!::e?1 Flank} 1.,,-, I Reef ff!#.f@U Crlnoidal-bar facies '-2 Core facies ~ Interreef facies I, Jtllnitial A

D

c E

Figure 6-17. Composition and interpretation of reef and interreef lithofacies in the Lockport Dolomite of northwestern Pennsylvania. A. Schematic diagram showing the relative position of a reef-core, reef-flank, and interreef facies in the lower Lockport Dolomite (from Crowley, ~1973, Figure 12, p. 291, reprinted by permission of the American Association of Petroleum Geologists). B. Stromatoporoid in the reef-core facies of the Lockport Dolomite, G. W. Snyder well core, Mer­ cer County. C. Stromatoporoid and coral rubble in a dolomite matrix from the reef-flank facies of the Lockport Dolomite, G. W. Snyder well core, Mercer County. D. Rippled oolitic dolomite in the interreeffacies of the Lockport Dolomite, G. W. Snyder well core, Mercer County. E. Modern analog in the Florida Keys for the Lockport biostromal and biohermallitho­ facies. The patch reef is just to the left of the diver. The diver and the barracuda hover over reef rubble and carbonate sand. 104 C.D.LAUGHREY

stages in central Pennsylvania. The Wills 2 to 10 m thick Creek Formation consists of variegated clay­ stone, silty claystone, and argillaceous lime­ Limy to dolomitic, IIght·olive·gray mud·mud· stone. The thicknes~ of the Wills Creek ranges stone. Mud cracked. from 250 to 500 feet. The upper and lower SUPRA· contacts are gradational and conformable. TIDAL The Tonoloway Formation conformably , overlies the Wills Creek Formation. It con­ Umy grayish·red mudstone. Mud cracked. sists mainly of laminated to thin-bedded lime­ stone and a few thin beds of calcareous shale. HIGH INTER · InterlaminatedInterlaminaled limy clay shale and mimi·· Some thin to medium beds of dense micro­ TIDAL critic limestone (or microcrystallinemicrocrystalline dolo­ crystalline limestone also occur. Faill and stone). Mud cracked, especiallyespecialiy IInn upper lOW part. Wells (1974) reported the occurrence of pel­ INTER· -- TIDAL ------let textures, ostracode shells, lenses of crys­ Thin· to medium·bedded, commonly vug· talline calcite, and sedimentary boudinage SUB· gy dolostone, dolomitic limestone, and TIDAL micriticmicritic limestone, commonly weathered structures in the Tonoloway. to yellowish brown. Few ostracodes. Both the Wills Creek and Tonoloway Formations consist of numerous shallow­ ing-upward cycles that Lacey (1960), Tourek Figure 6-18. Sedimentary cycle in the Wills Creek Formation (1971), and Cotter and Inners (1986) have (from Cotter and Inners, 1986, Figure 16). interpreted as repeated progradational events on very large tidal-sabkha flats (Figures 6-18

and 6-19). 3 to 8 m thick Cyclicity in the Wills Creek-Tonoloway 0' c::::::t ~ ~ 0/ Thln- to medium-bedded,medium-bedded, commonly intemal· has been ascribed to both autogenic and al­ SUPRA. "/"""/""/-/<1"/ -/<1"/ Iy laminated, argillaceous, vuggy, microcrys­ TIDAL 7""/"""/-'" talline dolostone and dolomitic limestone, logenic mechanisms by different workers. containing salt casts. Locally brecciated. Tourek (1971) proposed that localized ba­ I' Cryptalgal laminates,nates, mud-cracked, com­ I' sin,al control on sedimentation is the princi­ HIGH monly dolomitic. INTER· I! pal mechanism governing the cyclicity ob­ TIDAL laminated, mud-eracked, micritic limestone, dolomitic toward top. Few ostracodes. ; 1 served in the sediments, whereas Anderson and Goodwin (1980) suggested eustatic con­ trol for the depositional events. Thin-beddedTh in-bedded to laminated micritic limestone, I~~~' .,....,--_ . ~~ commonly containing stromatolites 0.3: m In diameter; shallow mud cracks common. , I TIDAL Numerous ostracodes. ! I Western Pennsylvania­ j I I Salina Group Thln- to medlum·bedded, locally burrowed, micritic to areniticarenltic limestone, containing SUB· abundant ostracodes and a few other In­ The Salina Group in northwestern Penn­ TIDAL = ,....0 vertebrates. Locally intraclastlc. OoliticOolltfc In sylvania consists of interbedded carbonate ' 0 0 0 ""'" • •• ...J...--"'"" lower mladle part of formation. and evaporite rocks. It ranges in thickness from over 2,000 feet in the southeastern part of the Appalachian Plateaus area to less than Figure 6-19. Sedimentary cycle in the Tonoloway Formation 400 feet at Lake Erie. Correlation of the (from Cotter and Inners, 1986, Figure 16). Salina Group intervals with outcrop equiva- lents in central Pennsylvania is shown in Fig- ure 6-6- 20. Salt beds of Unit B in Figure 6-20 appear distribution of Unit B indicates that the development to be continuous from the Michigan basin into the Ap­ of a subbasin in north-central Pennsylvania was con­ palachian basin (Rickard, 1969). This unit contains trolled by the location of Niagaran reefs, the eastward the first known salt beds of the Salina deposited in restriction of the Bloomsburg delta, and a higher plat­ Pennsylvania. Rickard (1969, p.p. 8) stated that the in­ fonn area in southwestern Pennsylvania (Fergusson flux of terrigenous sediments of the Bloomsburg delta and Prather, 1968). The probable paleogeography of inhibited the deposition of evaporites in the Unit A the Salina salt basin during mid-Cayugan time is illus­ rocks (Figure 6-20) of the central Appalachians. The trated in Figure 6-21. CHAPTER 6-SILURIAN AND TRANSITION TO DEVONIAN 105

-. ", CEiTRALCEIITRAL AIDAIID) WESTERII'!IESTERM "lEWIEW lDRKlORK k.• 1 ~"~':~"~':.'.. ., ,f·:,f.: . , ' 111-.(11-. .••iod IUbnrfaca)IUbsarface} ,• . . ., III f''''''" _.' J._'._ ~ "_'!.. ii~ ~ !! AID WWORTH-CEllnlALORTH-CElnlAL PElISnVAl1APEllIISnVAlL1A , CEIIT1IA1.CEIITRAL PElISnVAIIAPUISnVAILIA '" i';,,(lubsurfaul(IUbsurfaul (Ricunl,(Ricbrd, 19691 ~. , .". . _ (.lIUnIp~(.uunp~ ~.r . ~r~;: Cobleskill-Akron Formation Keyser Illil'/''i1'/ Bertie Formation Formation Unit H . , ? .,.: " Camillus . Unit G I' ' ". Formation

·t~7'.. "". ". '"';"t. .,, ~,:~,~ c:: Unit F* .!.~ 'E '. .'.1 ... ------Tonoloway - E IiI~ " ., e> Formation u..u.= Unit E* ' c:: CD I -.~,:-:i~"-i ca c.Co :I::I I·f, il;0:"' :::I '" = C) ... ::I ::::I e ----- ...... ,1;- t5t!l e>- en ' u en ,~,: .5.. Unit D* 1I'('! 0:ii:.';:" a; ' LU a; D- enCIJ D- ::I Unit C =c:: I": .1 a 50 100 MI '" .. ;~ I": .1 .! W~ls Creek i..~ ca ------~I-'I;--LI--'I;--LI "'TI-.....,...J.""T-~i ' E Formation a 50 100 150 KM e> u..u.= " ..\' Unit B* .', c:: =e> (l =E Figure 6-21. General paleogeography of the central CD --- ., :>:>- Bloomsburg Appalachian region in mid-Cayugan time (from Cotter . ~ ,.• i,;c', ' ~ Unit A Formation and Inners, 1986, Figure 17).17) • , Formation :f;'l:~'l Ipart) ., ·Salt beds in Pennsylvania of the Andreas is uncertain; it may be correlative with the upper part of the Decker Fonnation or the lower­ Figure 6-20. Correlation of evaporite-bearing intervals in the Salina Group with outcrop equivalents in central most Devonian Rondout Formation (Lash and others, Pennsylvania (from Cotter and hmers, 1986, Figure 18). 1984; Berg, McInerney, and others, 1986). The Keyser Formation in eastern Pennsylvania is made up of approximately 125 feet of gray, argilla­ UPPER PRIDOLIAN-LOWEST ceous, fossiliferous, nodular limestone and some in­ DEVONIAN terbedded calcareous shale (Inners, 1981). Basal and upper contacts are confonnable. Inners (1981) inter­ Eastern Pennsylvania-Keyser and preted the lower two thirds of the Keyser in eastern Decker Formations Pennsylvania as having fonnedformed in a shallow-marine, The Decker Fonnation is the youngest undisputed subtidal shelf environment. Deposition of the upper Silurian unit in northeastern Pennsylvania (Epstein third of the fonnation was in shallow lagoons and on and others, 1974)1974).. Conodonts collected by Denkler intertidal mudflats, similar to that of the subjacent (1984) confirm the Pridolian (youngest Silurian) age Tonoloway FonnationFormation (Inners, 1981). Silurian fossils of the Decker Fonnation. The interval is 80 to 90 feet occur in the lower part of the Keyser, whereas Devo­ thick in northeastern Pennsylvania and consists of are­ nian fossils occur in the upper part, demonstrating naceous limestone and some argillaceous siltstone that the systemic boundary lies within this formation and sandstone. Fossils include ostracodes, brachio­ (Berdan, 1964; BowenBowen,, 1967). podspods,, bryozoans, stromatolites, and conodonts. The Decker formed as barrier beach andlorand/or biostromal Central Pennsylvania-Keyser Formation banks (Epstein and others, 1967) and thins to the The Keyser Formation of central Pennsylvania southwest. The upper and lower contacts are conform­ represents continuous carbonate sedimentation from able. In some parts of northeastern Pennsylvania, the Late Silurian into Early Devonian time. Both Silurian Decker is overlain by the Andreas Red BedsBeds.. The age and Devonian fossils occur in the Keyser (Bowen, 106 C.D.LAUGHREY

>- '0 '0 '0 .., iii e e e e .c 0 OUI 0 OUI .,., .., 0.., 0.- 0. 0._ .! .!., ::I., ~ ~ en ., 0- Oc oc", BE g"E "E =±: Co cOle OJ -c 010 Se lii UI_ c: UI -!.~; ,Cii! ~~ :; CDco.2 .., e::: -"-~i u~ a"' .g'~~ E c: 0" !o.t: ~~~ "'C "E-g 01 OIl ::..s! ~e.2 0~0 iii ~:2 0 UlOI FOSSILS ...I III/JJ UJ~ (.)1ij~ 0-;;:0 8~~ 0 oB z ~., .!t~~t~

Corals

Articulate brachiopods

BryoloanBryoloans

Crinoids

Cystolds ? ? ? ? ? ?

Molluscs

Pelecypods

Gastropods ----~

Cephalopods

Tentaculltlds

TrilobTrilobitesites

08tracodesOstracodes II Algae

Conodonts ? ? ?

" Burrowers" ------

Superimposed on IntertidalIntertidal and very BiohermslBiohermal subtidal GEGENERALNERAL , I other facfaciesies ENVIRON· shallow subtidal MENT Inter· Inter· biohermal biohermal 1------Generally seaseawardward Generally seaward

Figure 6-22. Head's (1969, Figure 33) interpretation of environmental relationships between lithofacies, fossils, and the sedimentary depositional setting of the Keyser Formation.

! -"- CHAPTER 6-SILURIAN AND TRANSITION TO DEVONIAN 107

1967}. The formation is recognized throughout the ing." A few of the unresolved subjects, however, war­ central Appalachian basin. The Keyser is a mainly rant special mention. The provenance of the frame­ gray, fossiliferous limestone. The upper part of the work constituents in the sandstones and conglomer­ fonnation consists of laminated to thin-bedded lime­ ates of the Shawangunk Formation is enigmatic (Ep­ stone and dark-gray chert nodules. The rest of the stein and Epstein, 1972)1972).. Considerable work is still formation is thin to very thick bedded. Stylolites needed with regard to cyclicity and correlation in the commonly parallel the bedding. Fossils are typically, Medina and Tuscarora intervals. The origin of chan­ but not always, disarticulated. Fragments of brachio­ nel deposits in the Grimsby Sandstone and the pos­ pods, crinoids, bryozoans, molluscs, and ostracodes sible fluvial nature of the lower part of the Whirlpool are common, but unbroken specimens occur (Hoskins Sandstone deserve careful attention (Duke and Brusse, and others, 1983)1983).. The reported thickness of the 1987; Middleton and others, 1987). Silurian carbon­ Keyser ranges from 75 to 202 feet. The lower con­ ates are mostly dolomitized in the subsurface of west­ tact of the Keyser with the Tonoloway is sharp and ern Pennsylvania, whereas mostly limestones occur conformable. The upper contacts with Lower Devo­ in the outcrop belt of central Pennsylvania. The dif­ nian lithologies are less distinctive. In east.:.central ferences in the diagenetic history of these rocks would Pennsylvania, the upper Keyser limestones grade up­ make an excellent research project. Further resolution ward into cherty limestone and shale, which in turn of the mechanisms controlling cyclicity in the Wills grade upward into the Devonian Old Port Forma­ Creek-Tonoloway interval is needed. Finally, detailed tion. To the west, the top of the Keyser Formation correlation and stratigraphy of the interval between is marked by a distinct chert bed (Conlin and Hoskins, the top of the Salina Group and the base of the Onon­ 1962). Head (1969) described the variations in ma­ daga Group would improve our understanding of the rine sedimentary environments that existed in the subsurface Upper Silurian and Lower Devonian rocks central Appalachians during deposition of the Keyser in western Pennsylvania. Formation (Figure 6-22). RECOMMENDED FOR FURTHER Western Pennsylvania-Keyser Formation READING and Equivalents Berdan. 1. M. (1964). The Helderberg Group and the position of In west-central Pennsylvania, the Keyser For­ the Silurian-Devonian boundary in North America, U.S. Geo­ mation is recognized in the subsurface of the Appa­ logical Survey Bulletin 1180-B. 19 p.p. lachian Plateaus province (Heyman, 1977). Farther Cotter, Edward (1982), Tuscarora Formation of Pennsylvania, west and northwest, the lower part of the Keyser is Society of Economic Paleontologists and Mineralogists, Eastern Section. 1982. Guidebook, 105 p. equivalent to the Silurian Bertie Dolomite, Akron and ___ (1983), Shelf, paralic. and fluvial environments and Cobleskill Dolomites, and Bass Islands Dolomite. eustatic sea-level fluctuations in the origin of the Tuscarora For­ The uppermost part of the Keyser is equivalent to mation (Lower Silurian) of central Pennsylvania, Journal of Sedi­ the Lower Devonian Manlius Formation. In extreme mentary Petrology. v. 53, p. 25-49. nQrthwestern Pennsylvania. the basal lithologies of Epstein, I. B., and Epstein. A. G. (1972), The Shawangunk For­ mation (Upper Ordovician(?J to Middle Silurian) in eastern Penn­ the Devonian Onondaga' and Oriskany Formations sylvania. U.S. Geological Survey Professional Paper 744.45 p. lie directly on the Silurian rocks. This interval, from Fergusson. W. B.•. • and Prather. B. A.A. (1968), Salt deposits in the the base of the Devonian Onondaga Group to the Salina Group in Pennsylvania, Pennsylvania Geological Sur­ top of the Salina Group, is one of very abrupt lithic vey. 4th ser., Mineral Resource Report 58.41 p. changes (Heyman, 1977). Hoskins, D. M. (1961), Stratigraphy and paleontology of the Bloomsburg Formation of Pennsylvania and adjacent states, Pennsylvania Geological Survey. 4th ser., General Geology Report 36, 125 p.p. PROBLEMS AND FUTURE Laughrey, C. D. (1984), Petrology and reservoir ciuJracterisrics of RESEARCH the Lower Silurian Medina Group sandstones, Athens and Geneva fields, Crawford County, PennsylvaniaPennsylvania,, Pennsylvania Geological A number of challenging problems and topics Survey. 4th ser.,., Mineral Resource Report 85. 126 p. for further study await the geologist interested in Silu­ Smosna, Richard, and Patchen, Douglas (1978), Silurian evolution rian sedimentation and stratigraphy in Pennsylvania. of central Appalachian basin, AAPG Bulletin, v. 62, p.p. 2308- 2328. A review of these topics is beyond the scope of this Yeakel, L. S., IT. (1962), Tuscarora, Juniata, and Bald Eagle article, and those who are curious should read publi­ paleocurrents and paleogeography in the central Appalachians, cations listed in "Recommended For Further Read- Geological Society of America Bulletin, v. 73, p. 1515-1540.