Erosion on an anaerobic seafloor: Significance of reworked pyrite deposits from the Devonian of New York state

Palaeogeography, Palaeoclimatology, Palaeoecology, 57 (1986): 157-193

157

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

EROSION ON AN A N A E R O B I C SEAFLOOR: SIGNIFICANCE OF R E W O R K E D PYRITE D E P O S I T S FROM THE D E V O N I A N OF N E W YORK S T A T E

GORDON C. BAIRD and CARLTON E. BRETT

Department of Geosciences, State University of New York College, Fredonia, N Y 14063 (U.S.A.) Department of Geological Sciences, University of Rochester, Rochester, N Y 14627 (U.S.A.) (Received August 14, 1985; revised and accepted April 23, 1986)

ABSTRACT Baird, G.C. and Brett, C.E., 1986. Erosion on an anaerobic seafloor: significance of reworked pyrite deposits from the Devonian of New York State. Palaeogeogr., Palaeoclimatol., Palaeoecol., 57: 157-193. Several Devonian black shale units in New York State display conspicuous basal discontinuities marked by accumulations of exhumed and reworked pyrite nodules, tubes, and fossil steinkerns. This coarse debris is overlain by, and/or interlayered with, black, laminated shale. The most important level is the Leicester Pyrite Member, which occurs along a regional unconformity flooring black shale deposits of the basal Genesee Formation; the detrital pyrite occurs in discontinuous lenses both on, and slightly above this surface. Basal Genesee beds (Geneseo Shale Member) record westward overlap of basinal, anoxic muds during a major marine transgression (Taghanic onlap). The reworked pyrite deposit is regionally diachronous, as indicated by conodont data and shingling of lenses with basal Genesee beds which are successively younger from east to west. Pyrite, chemically unstable in aerobic bottom settings, was exhumed and concentrated on the sea floor during brief erosion episodes in a normally anaerobic environment. Lenses contain distinctive diagenetic pyrite structures (e.g., fossil steinkerns, burrow tubes) derived from muds underlying the discontinuities. Erosional reworking of pyrite on the sea floor is shown by mechanical breakage of pyrite grains, reorientation of geopetal stalactitic pyrite and compactional features, plus alignment of pyritic tubes by bottom currents. General absence of carbonate allochems in lenses is believed to reflect dissolution of carbonate, following its exhumation. Episodic, deep storm-generated turbulence was probably important in exhuming and transporting pyrite. However, shoaling internal waves, impinging the sediment-starved basin margin slope, may also have been important in scouring the bottom. A model of internal wave erosion during the Taghanic transgression is presented herein; we feel t h a t such erosion, concentrated at or near the base of the pycnocline during a period of relative sea level rise, offers a particularly good explanation for the observed juxtaposition of Geneseo and other similar black shale deposits on erosional discontinuities. The diachronous character of Leicester lenses and associated black Genesee muds above the unconformity suggests a t r a n s i e n t r a t h e r t h a n fixed basin axis allowing for progressive westward burial of the unconformity and associated remani~ deposits by onlapping black muds. Older and younger occurrences of resedimented pyrite are briefly described from other Devonian basinal units and are shown to have formed in about the same m a n n e r as the Leicester. Reworked pyrite is believed to be common, although seldom recognized, in Phanerozoic sediment-

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© 1986 Elsevier Science Publishers B.V.

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starved basinal settings. Moreover, detrital pyrite, analogous to that described herein, probably awaits discovery in certain modern marine basins. INTRODUCTION Various types of transported and resedimented debris occur in areas of submarine erosion on the outer continental shelf, often at or below the slope break. Sand, shells, and, commonly bone, are concentrated as gravel and sand sheets or as starved ripples on the sea bed (Hollister and Heezen, 1972). Holocene relict sediments include diagenetic phosphorite and glauconite (Emery, 1968; Stanley and Wear, 1978; Stanley et al., 1983). Similarly, remani~ deposits on ancient unconformities are characterized by concentrations of the above materials, calcareous skeletal allochems, and occasionally oolitic hematite. As is demonstrated herein, submarine discontinuities, given the proper paleoenvironmental setting, also can yield placer lags of reworked pyrite. In this paper we discuss submarine erosion in a large Devonian foreland basin, and specifically, the discovery of extensive deposits of reworked sedimentary pyrite along discontinuity surfaces overlain by anoxic to dysoxic pelagic sediments. Such pyrite occurs as nodules, fossil steinkerns, and pyritized tubular burrow sheaths on several different discontinuity surfaces. Stratinomic evidence indicates that these pyritic clasts were derived from subjacent muds and we present several criteria for distinguishing reworked from in-situ pyrite in the stratigraphic record. The present study indicates that exhumation and mechanical transport of iron sulfide was widespread in a setting which was predominantly anaerobic, and that strong bottom currents, generated by episodic deep-storm turbulence, and possibly by impinging internal waves, produced this erosion. This bottom erosion produced widespread disconformities and associated pyrite concentrations. Relict sediment, originally composed of both carbonate and pyritic grains, was apparently eroded from a gently-inclined, mud-floored submarine slope; a model is presented to explain erosion of this debris, its subsequent downslope transport, and its final attrition by dissolution to a pyrite and bone residue before burial by basinal muds. Diachronous burial of the major Taghanic Unconformity occurred as a result of basin-filling and tectonic adjustment. A model is proposed for the discontinuous generation of Leicester-type reworked pyrite along this and other, minor disconformities during latest Givetian to medial Frasnian time, as the tectonically transient Appalachian Basin was filled with sediment. GEOLOGIC SETTING

Paleogeography and tectonic setting The Leicester Pyrite and associated Hamilton and Genesee deposits are components of a thick and largely terrigenous sequence within the Appala-

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Fig.1. Devonian paleogeography and study area. A. Middle Devonian (Givetian) paleogeography based on reconstruction of Seyfert and Sirkin (1979); stippled area submerged. B. Detail within A showing position of New York relative to major paleogeographic features. C. Study area. Heavy line denotes outcrop of Leicester Pyrite Member and equivalent deposits. Limestone symbols show extent of Tully Member. Dots along outcrop belt mark unlisted Leicester sections. Numbers denote outcrops described in Appendix I. 1, ~. North Evans Limestone Member. 3-10, 11, 14, 16: Leicester and Leicester eastern equivalents. 12, 13: pyrite at Lodi Limestone horizon. 15: pyrite occurrence in siltstone bed below Lodi.

chian Basin of eastern North America. Sediments in the study area (Fig.l) a c c u m u l a t e d a t t h e n o r t h e r n m a r g i n o f t h i s b a s i n a b o u t 10 t o 15 ° s o u t h o f t h e i n f e r r e d p a l e o e q u a t o r ( F i g . l . A , B) b a s e d o n p a l e o m a g n e t i c r e c o n s t r u c t i o n s ( S e y f e r t a n d S i r k i n , 1979). C o n v e r g e n t a c t i v i t y i n v o l v i n g o b l i q u e c o l l i s i o n o f o n e o r m o r e s u b c o n t i n e n t s ( A v a l o n i a ) a n d p o s s i b l y a n i s l a n d a r c is r e s p o n s i b l e for the extensive Acadian orogeny which produced mountains in New England

160 and the eastern/central Atlantic states (Dewey and Burke, 1974; Dewey and Kidd, 1974; Van der Voo, 1982). Initial Acadian disturbance in New York State is marked by abrupt upward change from carbonate deposits to largely terrigenous sediments of the Middle Devonian Hamilton Group which records erosion of the rising mountains to the east (Cooper, 1957; Rickard, 1981). However, a major pulse of orogenic activity is associated with post-Hamilton deposits; this is reflected in regional crustal diastrophism, locally high sedimentation rates, and widespread transgression in the latest Givetian (Friedman and Johnson, 1968; Johnson and Friedman, 1969; Johnson, 1970; Heckel, 1973; Dennison and Head, 1975). Hamilton shales and a post-Hamilton carbonate unit, the Tully Limestone, are abruptly succeeded by black shale of the Genesee Formation in western New York (Heckel, 1973; DeWitt and Colton, 1978). This boundary is a widespread erosional discontinuity which is overlain by basinal black shale deposits of latest Givetian to Famennian age over a ten-state region (Conant and Swanson, 1961; Dennison and Head, 1975; Cluff, 1980). For purposes of discussion herein, we refer to this regional discontinuity as the Taghanic Unconformity because this break terminates eastward into stratigraphic continuity within the Late Middle Devonian Taghanic stage of North America (see Johnson, 1970). The Genesee Formation and succeeding clastic divisions of the Upper Devonian record rapid growth of the Catskill Delta, a large tectonic delta system bounded by a sublittoral to upper bathyal basin to the west and northwest (Sutton et al., 1970; Thayer, 1974; Broadhead et al., 1982). The delta complex prograded through the duration of the Frasnian and Famennian, eventually filling the basin in the late Famennian. The deepest part of the basin during Genesee time was probably centered in western Pennsylvania (Fig.l.B) south of the study area (McIver, 1970; Dennison and Head, 1975; Lundegard et al., 1980), but the basin axis apparently shifted westward through processes of tectonic adjustment and isostatic effects during the Late Devonian (Dennison, 1983; Ettensohn, 1985). The northern and western boundaries of the basin bordered low-relief, cratonic shelf regions. These areas supplied relatively little detrital sediment as compared with actively rising tectonic source terrains to the southeast, resulting in periods of sedimentstarvation on this slope. The reduced sediment supply accounts for thin Devonian deposits and widespread discontinuities in Ontario, Ohio, and westernmost New York (Dennison and Head, 1975; Rickard, 1975).

Depositional setting and facies Shales and thin limestones of the Hamilton Group, below the Leicester pyrite, are characteristically richly fossiliferous and record stable, subtidal shelf settings across central and western New York (Cooper, 1957; Baird and Brett, 1983). The Hamilton sea was apparently shallow and had near-normal salinity, water temperatures, and circulation as shown by the diverse benthos.

161 The overlying Tully Limestone, developed in central and west-central New York, likewise records aerobic infralittoral conditions as is indicated by abundant shelled organisms at many levels within this unit (see Heckel, 1973). The succeeding Genesee Formation contains an eastward-changing facies spectrum from basinal black shale deposits through prodelta slope facies, characterized by numerous turbidites, to delta platform and nearshore deposits composed mainly of fossiliferous siltstones and sandstones (Thayer, 1974). From Erie County eastward into central New York the Genesee sequence increases more than one hundred-fold in thickness as basinal deposits pass laterally into slope and platform facies. Three meters (10 ft) of Genesee deposits at Lake Erie (Loc. 1) 1 expands eastward to approximately 365 m (1200 ft) at Ithaca (DeWitt and Colton, 1978). GENESEE BASINALDEPOSITS: PALEOBATHYMETRY-SUBSTRATECONDITIONS Conant and Swanson (1961) in a major study of the Chattanooga Shale, summed up several problematical features of this unit, which have been the subject of long-standing debate; these include: (1) juxtaposition of black shale facies upon a widespread unconformity; (2) lateral overlap of Chattanooga beds such that most parts of the formation rest on the basal unconformity in one or more parts of the study area; (3) close association of Chattanooga black shale beds with a thin basal sandstone, or coarse lag debris layer on the unconformity; and (4) the occurrence of numerous thin, current-rippled siltstone beds within the laminar, black shale facies. Citing these observations, Conant and Swanson (1961) argued that Chattanooga muds accumulated in a shallow (upper infralittoral) marine environment. Subsequent studies of Devonian black shale units by other workers have generally led to deeper-water models for deposition of this facies; Genesee and overlying Sonyea black shales are interpreted as representing "outer shelf and basin" conditions (Bowen et al., 1974; Thayer, 1974). These units, as well as younger black shales, are given depth values of up to 230 m (700 ft) in northwest Pennsylvania and northern Ohio (Broadhead et al., 1982). Parts of the Braillier Formation in Pennsylvania, to which the Genesee is partially equivalent, are believed to have been deposited in water up to 530 m (1600 ft) in depth (Lundegard et al., 1980). Evidence presented here suggests that Genesee black shale deposits record deposition in the outer sublittoral zone or deeper. Firstly, post-Tully black shale units are very widespread. The Geneseo, Penn Yan, and West River black and dark gray shale members of the Genesee Formation are facies coextensive with the Braillier Formation in Pennsylvania, parts of the Ohio and Antrim shales in Ohio and Michigan, and the New Albany and Chattanooga shales in the eastern midwest and Appalachian regions respectively

1Localities numbered in text and figures are described in Appendix I.

162 (Dennison and Head, 1975; Cluff, 1980; Conkin et al., 1980; Ettensohn, 1985). Even though parts of these widespread deposits are chronologically younger than Genesee, this facies is spatially continuous. It is unlikely that shallow subtidal conditions would sustain anaerobic or dysaerobic conditions over so large an area for so much time. In addition, the Genesee and Braillier formations grade eastward through thick sequences of turbiditic siltstone into the fossiliferous delta platform facies (Thayer, 1974; DeWitt and Colton, 1978). This facies spectrum indicates the presence of a significant west-facing basin margin slope. The paleoenvironment of the basal Genesee sequence (Geneseo Shale Member and associated Leicester Pyrite) is interpreted as a dysaerobic to anaerobic basin with a variably stratified water column (see model of Byers, 1977, for basal Sonyea black shales). Density stratification may have been maintained, in part, as a halocline resulting from incursion of lighter fresh waters from the eastern delta (Thayer, 1974; Byers, 1977). Such stratification may have produced a "silled basin" effect (e.g. Byers model) with a strictly horizontal layering of water zones or a subhorizontal, vertically shifting pycnocline as proposed by Ettensohn and Elam (1985). The aforementioned features, cited by Conant and Swanson (1961), for their shallow-water interpretation of the Chattanooga are all observed in the Geneseo Member which contains essentially identical facies. We find that all these features, including a widespread unconformity at the base of the Geneseo, significant regional overlap of Geneseo beds, and evidence for strong current flow within the black shale basin, are consistent with a deeper-water interpretation of this unit, given recent oceanographic discoveries. Processes believed to have produced these features in a sloped basinal setting, are discussed in later sections.

Taghanic Unconformity The Taghanic Unconformity, along which the Leicester pyrite occurs, marks a major, widespread hiatus, regionally overlain by basinal black and olive gray shales across large parts of the eastern midwest and southern Appalachian regions. This lacuna is of variable time significance. Where it attains greatest magnitude, southwest of New York State (Rich, 1951; Allen and Friend, 1968; Broadhead et al., 1982), it reflects additive effects of erosional truncation of pre-Taghanic strata, juxtaposition of Upper Devonian basinal discontinuities, and delayed sedimentary onlap (see Brett and Baird, 1982, for discussion of westward convergence of Taghanic and a younger, subsidiary unconformity). Uppermost Tully deposits, coeval with the eastern limit of the Taghanic Unconformity record a facies transition into Genesee black shale in central New York (Heckel, 1973), while an abrupt change across this sedimentary break from Tully Limestone or Hamilton shales to black shale deposits is recorded further west (Heckel, 1973; Rickard, 1975; Brett and Baird, 1982). The

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Fig.2. East west Leicester stratigraphy and inferred chronostratigraphy. Note westward onlap of Genesee deposits over the Taghanic Unconformity (1). Information based partly on Rickard (1975) and Brett and Baird (1982). Vertical lines denote hiatus intervals. magnitude of this unconformity increases westward from its point of inception in the Skaneateles Valley (Figs.2 and 3) with progressive downward loss of underlying Tully and Hamilton strata in the westward direction (Cooper, 1930; Cooper and Williams, 1935; Brett and Baird, 1982). The Tully is conspicuously overstepped from Seneca Lake westward (Brett and Baird, 1982); this limestone occurs as a "feather edge" erosional remnant (Fig.3) beneath the Leicester at its westernmost exposure (Loc. 9) east of Canandaigua Lake. From the Canandaigua Valley westward to central Genesee County, successively older beds of the Windom Member are bevelled in a northwestward direction (Fig.2), such that half of this unit is absent near Batavia (Brett and Baird, 1982). In addition to westward regional overstep, there is progressive westward loss of overlying Genesee conodont zones from the oldest upwards due to the effect of diachronous depositional onlap of black muds (Huddle, 1981). Huddle (1981) ahd Klapper (1981) note that conodonts become progressively younger within basal Genesee deposits (Schmidtognathus hermanni-Polygnathus cristatus zone to lowermost Polygnathus asymmetricus subzone) from the Cayuga Valley westward to Buffalo. Stratigraphic observations support Huddle's conclusions that the base of the Genesee is diachronous; in western Erie County the base of the formation is marked by the basal Frasnian Genundewa Limestone Member (Figs.2 and 3); in eastern Erie and Genesee counties it is marked by the older Geneseo Member which progressively thickens eastward (Kirchgasser, 1973; Rickard, 1975; DeWitt and Colton, 1978). Westward depositional onlap of the Taghanic discontinuity is revealed through detailed mapping; from Genesee County westward, numerous thin, rhythmic, concretionary limestones within the

164

165 G e n e s e o M e m b e r of t h e l o w e r G e n e s e e F o r m a t i o n a r e o b s e r v e d to lap o n t o t h e T a g h a n i c b o u n d a r y ; t h e s e beds d i s a p p e a r f r o m t h e oldest u p w a r d s as the G e n e s e o t h i n s w e s t w a r d (Figs.2 a n d 3). T h i s o n l a p i n d i c a t e s t h a t a gentle, s o u t h e a s t e r l y - s l o p i n g b a s i n m a r g i n w a s p r e s e n t in w e s t e r n N e w Y o r k ( B r e t t a n d Baird, 1982). I t s u g g e s t s f u r t h e r w e s t w a r d s h o a l i n g b e y o n d t h e limit of m u d d e p o s i t i o n a n d a p r o b a b l e g r a d i e n t of i n c r e a s i n g e n e r g y c r a t o n w a r d . T h i s p a l e o s l o p e was p r o b a b l y p r e s e n t t h r o u g h o u t t h e l a t e s t Middle- a n d U p p e r D e v o n i a n , a l t h o u g h its p o s i t i o n shifted w e s t w a r d t h r o u g h t i m e (Dennison, 1983; E t t e n s o h n , 1985). T h i s o n l a p is on a t e m p o r a l scale of a t l e a s t o n e h a l f million y e a r s a c r o s s N e w Y o r k S t a t e alone; in w e s t - c e n t r a l N e w Y o r k , b l a c k s h a l e deposits a b o v e t h e b r e a k a r e l a t e s t G i v e t i a n ; a t Buffalo, t h e y a r e e a r l i e s t F r a s n i a n (basal P. a s y m m e t r i c u s zone), in s o u t h e r n O n t a r i o , t h e b a s a l b l a c k s h a l e s h a v e a middle P. a s y m m e t r i c u s a g e ( U y e n o et al., 1982), a n d in n o r t h w e s t Ohio, s u c h shales a r e s t r a t i g r a p h i c a l l y c o r r e l a t i v e w i t h beds m a r k i n g t h e top of t h e P. a s y m m e t r i c u s zone of t h e m e d i a l F r a s n i a n (Rickard, 1975; B r o a d h e a d et al., 1982). T h e r e g i o n a l a n d p a l e o g e o g r a p h i c a l s i g n i f i c a n c e of t h e T a g h a n i c U n c o n f o r m i t y is t h u s threefold. Firstly, it is of g r e a t g e o g r a p h i c a l extent, b e i n g m a p p a b l e a c r o s s p a r t s of all of t e n e a s t e r n U.S. states; secondly, it is o v e r l a i n a c r o s s m o s t of this a r e a by b a s i n a l p e l a g i c s e d i m e n t s and, thirdly, s h a l e deposits r e s t i n g on t h e u n c o n f o r m i t y differ in a g e by as m u c h as f o u r million y e a r s w i t h i n e a s t e r n North America. LEICESTER PYRITE: REGIONAL CHARACTER AND MODE OF FORMATION General character and regional variation of pyrite lenses

T h e L e i c e s t e r P y r i t e M e m b e r , f o r m a l l y d e s c r i b e d by S u t t o n (1951), i n c l u d e s d i s c o n t i n u o u s lenses of p y r i t i c clasts a n d fish b o n e s w h i c h o c c u r at, or v e r y s l i g h t l y above, t h e u n c o n f o r m a b l e b a s e of t h e G e n e s e o S h a l e M e m b e r (Ful-

Fig.3. Stratigraphy of Leicester Pyrite and associated sediments. A. Detailed Windom-Genesee stratigraphy along NE-SW-trending profile in Erie County. Note shingling and southwestward onlap of Leicester lenses above Taghanic discontinuity; northeastward erosional overstep of Windom strata below Genesee deposits; southwestward erosional overstep of Windom and lower Genesee strata below sub-Genundewa discontinuity. Units include: a = Buffalo Creek Shell Bed; b =Penn Dixie Pyrite Bed; c=Amsdell Limestone Bed; d = Leicester Pyrite Member; e = Lower Genesee shale succession; f= North Evans Limestone Bed (remani~ material); g= Genundewa Limestone Member. Note great vertical exaggeration of scale. B. Eastward termination of Taghanic Unconformity. Sections a (Loc. 9) and b (Loc. 11) show conspicuous truncation of Tully Formation below hiatus. Sections c (Loc. 14) and d (Loc. 15) in Cayuga Valley show reduction in magnitude of hiatus with eastward appearance of a transitional dysoxic limestone sequence (Fillmore Glen Beds). Note local angularity of Taghanic discontinuity at Section B. Units include: l=Windom Shale Member; 2=basal Tully carbonates; 3=Taughannock Falls Bed (Tully); 4= Bellona Coral Bed (Tully); 5= Moravia Bed (Tully); 6= Tsg~anic Unconformity; 7 = Leicester Pyrite Member; 8= Leicester-equivalent calcareous remani~ material at base o f - - and within - Genesee Member; 9= Fillmore Glen beds; 10= Genesee Member (black shale).

166

Fig.4. Leicester Pyrite lenses: general character and mode of occurrence. A. Starved Leicester lenses at Cazenovia Creek (Loc. 3). Units include: a = Windom Member; b = Leicester Member; c = lower Genesee black shale; d = Genundewa limestone Member. Photo width 11 m. B. Close-up view of Leicester at Loc. 3. Units include: a=Windom Member; b = Leicester lens; c-Lower Genesee black shale. C. Top surface of Leicester lens showing current-aligned tubular pyritic grains (Loc. 7). D. Multiple Leicester lens-sheet deposits with intervening black Geneseo shale (Loc. 6). Arrow denotes Windom Member with in-situ pyrite. Vertical cut section. Bar for C, D is 1.0 cm. All specimens in figures kept in University of Rochester Taphonomy Collection. reader, 1957; B r e t t a n d Baird, 1982; B a i r d and Brett, 1985). Lenses c o n s i s t i n g a l m o s t e x c l u s i v e l y o f p y r i t i c g r a i n s are t y p i c a l of this b o u n d a r y w h e r e G e n e s e o b l a c k s h a l e d i r e c t l y overlies v a r i a b l y fossiliferous, m e d i u m a n d d a r k g r a y shale of the W i n d o m M e m b e r of the M o s c o w F o r m a t i o n (Figs.2-4); lenses o c c u r a l o n g a 120 k m (70 mi) o u t c r o p line b e t w e e n C a z e n o v i a C r e e k (Loc. 3) in c e n t r a l Erie C o u n t y a n d F l i n t Creek (Loc. 10) east of C a n a n d a i g u a L a k e

167

(Fig.l.B), where the pyrite rests on the upper surface of the Tully Limestone. East of Flint Creek the remani~ debris becomes predominantly calcareous, but distinctly less abundant as the hiatus terminates eastward. In central New York, shell-rich beds of the uppermost Tully grade upward continuously

Fig.5. Content and Shape of Leicester lenses. A. Pyrite-clast-filled sublinear groove casts (microchannels) on base of Leicester lens (Loc. 3). Groove casts bevelled and polished for visual enhancement. B. Quartz pebble dropstone? (arrow) in Leicester lens (Loc. 7). Vertical cut section. C. Aligned t u b u l a r pyritic grains on top surface of Leicester lens (Loc. 7). D. Doubly-stacked lens separated by Geneseo Shale. Note profiles of scour troughs (microchannels) at Windom-Geneseo contact (Loc. 3). a = W i n d o m Member; b = G e n e s e o black shale; c = Leicester Pyrite. Vertical polished section. E. Vertical photomicrograph section of Leicester microchannel-filling. Note conspicuous grainstone texture of Leicester (Loc. 3). Letters denote: a = W i n d o m Member; b = Leicester Member; c = Geneseo black shale. F. Microchannel filled with pelmatozoan calcarenite. Basal Geneseo Member (Loc. 11). Vertical polished section. Bar for A is 1.25 cm; B-D, F is 1.0 cm.

168

through dysoxic carbonate deposits (Fillmore Glen beds) into Geneseo black shale (Figs.2 and 3.B). West of Cazenovia Creek, the Leicester and associated basal Genesee discontinuity are overstepped by the North Evans erosion surface (Brett and Baird, 1982; Figs.2 and 3). Leicester pyrite lenses are composed almost entirely of round to irregular pyritic grains with a subsidiary, bone, conodont, and quartz pebble component (Figs.4.A, B and 5.B). Most lenses are 2-6 cm (0.2-2.5 in.) thick; maximum lens thickness is 28 cm (11 in.) at Durkee Creek (Loc. 4) and maximum apparent lens length is approximately 8 m (25 ft), the average width-to-length ratio of lenses being about 1:6. The spatial geometry of Leicester lenses is difficult to reconstruct; it is impossible to see entire lenses as they can seldom be viewed from other than a planar outcrop face. Moreover, there are no foreset structures in lenses to confirm a starved ripple or gravel-wave mode of transport. However, detailed study of more than 15 lenses along a long curving outcrop face at Cazenovia Creek in Erie County (Loc. 3) shows that lenses are discontinuous in the long direction unlike wave-forms and have an irregular flute-shape geometry. Although most lenses occur on the Windom-Geneseo contact, a few occur a centimeter or more above it, within laminated black shale (Fig.4.D). Where sorting in lenses is observed it usually occurs as diffuse horizontal bands which sometimes display crude inverse-grading. This grading, however, may be an artifact of partial hydraulic scour of a preexisting pyritic lens such that a layer of coarse lag grains is left over finer ones. Moreover, since large grains on the tops of lenses are tubular and discoidal, this apparent grading may simply reflect shape-sorting. Pyrite lenses commonly display thin, horizontal black shale partings (Figs.4.D, 5.D and 6.G); these laminae demonstrate that lenses are amalgamations of pyrite-clast packstone or grainstone layers, each representing a discrete depositional episode. Erosional energy was expended during pyrite transport; at most outcrops rip-up clasts of Windom gray shale and, locally, Geneseo-type black shale occur in lenses. The sharp-edged shale flakes and larger tabular pieces attest to significant dewatering and compaction of underlying sediments; no sedimentary load structures have been reported under Leicester lenses or along the discontinuity. Parallel-aligned tubular pyritic clasts are the most common evidence of current fabric (Figs.4.C and 5.C); alignment has been observed not only in the Leicester but also in three younger pyrite horizons. The bases of Leicester lenses display sole marks which represent pyrite debris-filled grooves cut into underlying Windom sediments (Fig.5.A, D, E). These 3-8 cm-wide, subparallel grooves reveal bidirectional aspects of current flow; azimuths trend N E - S W in the Canandaigua Valley region but become more N-S in Erie County (Fulreader, 1957; this paper). Both grooves and aligned tubular grains are found to be parallel or subparallel to the long axes of lenses at Cazenovia Creek. Fulreader (1957) interpreted these sole marks as casts of ripples on the upper Windom surface. However, there is good evidence that Windom muds were

169

P,

B

Fig.6. Leicester Pyrite clasts. A. Prefossilized pyritic mold of orthoconic cephalopod. Chamberlining pyritization followed compression of shell; exhumed compressed mold is upended in Leicester; lateral view of specimen (Loc. 6). B. Top view of specimen A. Note horizontal fracture line. C. Broken pyritic steinkern of Tornoceras uniangulare (Loc. 8). D. pyritic mold of Paleoneilo sp. (Loc. 8). E. Ptyctodont tritor (crushing tooth) (Loc. 8). F. Prefossilized brachiopod (Allanella? sp.); note overpyrite rim along commissure (Loc. 7). G. Vertical photomicrograph section of stacked pyrite-grainstone lenses separated by laminae of Geneseo Shale (Loc. 3). H. Pyritepermineralized pelmatozoan columnal and associated loosely packed Leicester clasts. Acid etched Leicester grainstone lens (Loc. 3). All bars 0.5 cm; A-D shown at actual size.

a l r e a d y c o h e r e n t a n d c l e a r l y n o t of a c o n s i s t e n c y to be rippled. T h i s i n t e r p r e t a t i o n is r e i n f o r c e d by d i s c o v e r y of t o o l - m a r k s i m p r e s s e d i n t o t h e top W i n d o m s u r f a c e a t one locality. T h e s e " r i p p l e " m a r k s a r e m o s t likely e r o s i o n a l s c o u r f e a t u r e s c u t into u n d e r l y i n g firm s e d i m e n t by g r a i n s a l t a t i o n . F l o o d (1983) r e c e n t l y d e s c r i b e d parallel, 1-5 m-wide, e r o s i o n a l d e p r e s s i o n s on t h e sea floor, w h i c h h e t e r m e d " f u r r o w s " ; as well as s m a l l e r (3-10 cm-wide) c h a n n e l s w h i c h he d e s i g n a t e d " m i n i a t u r e f u r r o w s " . F u r r o w s a r e o b s e r v e d in c o n t i n e n t a l slope a n d rise s e t t i n g s affected by c o n t o u r c u r r e n t s (Flood, 1983), b u t t h e y also c a n o c c u r in deep c o n t i n e n t a l w a t e r bodies s u c h as L a k e S u p e r i o r ( J o h n s o n et al., 1984). T h e g e o m e t r y of L e i c e s t e r lenses s u g g e s t s t h e possibility of p y r i t e - c l a s t t r a n s p o r t a n d d e p o s i t i o n in l i n e a r furrows; firstly, t h e b a s i n a l c h a r a c t e r of s u r r o u n d i n g s e d i m e n t s s u g g e s t s a n e n v i r o n m e n t well b e l o w m e a n w a v e base, w h i c h is w h e r e m o s t R e c e n t f u r r o w s a r e observed, and, secondly, t h e w i d t h s of lenses a r e in a c c o r d w i t h t h e size of m a n y o b s e r v e d f u r r o w s (Flood, 1983). We f a v o r s u c h a n i n t e r p r e t a t i o n , b u t this o r i g i n r e m a i n s u n p r o v e n . Lenses,

170

i::?:i!:¸:¸¸'

Fig.7: Evidence of submarine pyrite exhumation. A. Vertical longitudinal section of orthoconic nautiloid showing geopetal pyritic stalactites in voids as well as geopetal floor-filling of living chamber (from Dick, 1982). Penn Dixie Pyrite Bed of Windom Member (Loc. 2). B. In-situ pyritic burrow tube showing vertical pyrite stalactites extending down into spar-filled axial cavity. Lodi Bed. Vertical cut section (Loc. 13). C. Veneer of resedimented pyrite along discontinuity at top of basal Lodi mudstone bed. Note contrast between Lodi mudstone (burrowed) and Geneseo Shale Member (laminated). Polished vertical section (Loc. 12). D. Lodi-Geneseo discontinuity showing protrusion of pyritic burrow tube above discontinuity surface. Vertical polished section (Loc. 13). E. Bedding plane-view of top surface of pyrite remani~ material on Lodi-Geneseo discontinuity. Note spar-filled tubular clasts with pyritic geopetal stalactites in two different orientations; polished surface (Loc. 13). F. Broken pyritic burrow tube with sharp end truncation. Leicester Member. Horizontal cut section (Loc. 5). Bars for A-E are 1,0 cm; F is 0.5 cm. h o w e v e r , c l e a r l y a p p e a r to be debris-fills of s h a l l o w d e p r e s s i o n s on t h e u n c o n f o r m i t y surface. T h e s c o u r g r o o v e s on t h e b a s e s of lenses a p p e a r to be t h e r e s u l t of t h e s a m e c u r r e n t a c t i o n t h a t f o r m e d t h e l a r g e r depressions. T h e s e g r o o v e s r e s e m b l e F l o o d ' s m i n i a t u r e f u r r o w s w h i c h a r e p r o d u c e d by c h a n n e l i z e d flow o f c o a r s e s k e l e t a l debris. T h e d e n s e g r a v e l - g r a d e p y r i t i c debris, o n c e e n t r a i n e d , could c e r t a i n l y a c t to s c o u r t h e s u b s t r a t e i n t o a c o m p l e x s c u l p t u r e of m i c r o c h a n n e l s . S e d i m e n t d e p o s i t s in t h e m i c r o c h a n n e l s a r e e x c l u s i v e l y p y r i t e clasts a n d b o n e debris in t h e r e g i o n of L e i c e s t e r o c c u r r e n c e (Fig.5.A, D, E), but, in t h e S e n e c a L a k e region, m i c r o c h a n n e l - f i l l i n g s c o n s i s t o f p e l m a t o z o a n g r a i n s t o n e . T h i s c a l c a r e o u s m a t e r i a l o c c u r s b o t h in lenses a t t h e b a s e of t h e G e n e s e o a n d in m i c r o c h a n n e l s or s h e e t c o n c e n t r a t i o n s u p to 30 cm a b o v e it (Figs.3.B a n d 5.F). C l a s t s in L e i c e s t e r lenses a r e p r e d o m i n a n t l y c o a r s e sand- a n d gravel-size p a r t i c l e s , a r e in c o n t a c t w i t h o n e a n o t h e r a n d u s u a l l y a r e m o d e r a t e l y to p o o r l y s o r t e d ( F i g s . 5 . B - E a n d 6.G). I n t e r p a r t i c l e s p a c e is o c c u p i e d l a r g e l y b y m u d m a t r i x , p a r t i c u l a r l y in l o c a l i t i e s e a s t of t h e G e n e s e e Valley. I n E r i e C o u n t y

171 sections the intergranular space is entirely calcite spar and lens fillings are analogous to grainstones (Figs.5.E and 6.G). Pyritic grains in lenses include irregular, nodular, and distinctly tubular particles with a subsidiary component of pyritic fossil steinkerns and permineralized wood. Lumpy, nodular to irregular clasts generally are most abundant, but locally, tubular clasts predominate (Fig.5.C); the latter are straight to gently curved tubes often displaying a spar-filled axial cavity that extends the length of the grain. Tubes have a granular surface appearance, sometimes with longitudinal ridges on the exterior, but ends of tubes often have a smooth, curved or straight surface indicative of breakage (Fig.7.F).

Origin of Leicester bioclasts Most Leicester macrofossils are preserved as worn and abraded pyritic steinkerns which commonly are fragmental (Fig.6.A-D); steinkerns include pyrite-lined molds of orthoconic nautiloids, the goniatite Tornoceras uniangulare (Fig.6.C), bivalves, including Paleoneilo spp. (Fig.6.D), and other protobranchs, the gastropod Glyptotomaria sp., and a small spiriferid brachiopod "Alanella" tullius (Fig.6.F); see Loomis (1903), Fulreader (1957), for complete lists of Leicester taxa. Crinoid columnals are locally common (Fig.6.H), and blastoid thecae are reported from the Leicester (Loomis, 1903). Fish teeth, including lozenge-shaped crushing teeth of ptyctodonts (Fig.6.E), placoderm armor, and conodonts are sufficiently abundant such that the Leicester is everywhere a good bone bed. Leicester fossils are believed to have been derived largely from strata underlying the Taghanic unconformity. Earlier workers believed that the Leicester fauna was a unique assemblage or one variously allied in composition to that of the Tully Formation of the Finger Lakes Region (Loomis, 1903; Sutton, 1951; Fulreader, 1957; Heckel, 1973); for this reason the Leicester was originally termed "Tully Pyrite" (Loomis, 1903). Different fossils in the Leicester indicate varying zonal status for the unit; small brachiopods and a variety of Tornoceras uniangulare suggest a pre-Tully age (Cooper and Williams, 1935; Kirchgasser and House, 1981), while many conodonts indicate a post-Tully (Geneseo-age) position (Huddle, 1981). Reexamination of this biota (Brett and Baird, 1982; this paper) shows that many, if not most, identifiable Leicester fossils and microfossils have been eroded from the Hamilton and Tully. '~Alanella" tullius in Canandaigua, Bristol, and Honeoye Valley Leicester deposits (Fig.6.F) was derived from the uppermost Ambocoelia? praeumbona beds where Alanella is similarly pyritized. This shale interval, which also yields numerous pyritized Mourlonia, Paleoneilo and cephalopods, is overstepped by the Geneseo between the Honeoye Valley and the Genesee Valley; west of the Geneseo Valley Alanella-bearing shale is absent below the Leicester and shells of this brachiopod are absent from lenses (Brett and Baird, 1982). Pyrite-impregnated crinoid columnals, common in Windom beds below the Alanella-rich zone, appear in great numbers in Leicester lenses as these

172 beds are progressively truncated west of the Genesee Valley (Fig.6.H). Although not all fossils are Windom-derived, most clearly are so, owing to the explicit match-up of the bulk Leicester fauna with that of subjacent strata. Some vertebrate debris has a more problematic origin; some placoderm armor and cladodont teeth have been found in the Hamilton, but ptyctodont tritors (lozenge-shaped crushing teeth), which are abundant in the Leicester (Fig.6.E), are unknown in the Windom (Hussakoff and Bryant, 1918). Because these teeth are highly worn, they may reflect considerable lateral transport from an environment no longer retained in the rock record. Genesee-age exhumation of Windom conodonts was noted by Huddle (1981); conodonts from the middle Polygnathus varcus subzone (Windom) occur admixed with younger basal Geneseo elements. The fact that many Leicester lenses are completely enclosed by Geneseo shale, however, indicates that the youngest Leicester conodonts must be contemporaneous with this unit. Loomis (1903) described the fauna of the Leicester as diminutive and implied that it was a distinct dwarfed assemblage; comparison of these fossils with those in the underlying Hamilton Group (Fulreader, 1957; Brett and Baird, 1982) show the taxa to be essentially identical in size and to show no particular evidence for unusual or harsh paleoenvironmental conditions; most simply represent normally small taxa. Effects of current sorting of Leicester steinkerns, partial diagenetic destruction of chambered shells during pyritization, and differential destruction of large calcareous fossils probably accounts for the small size of Leicester fossils overall (Brett and Baird, 1982).

Exhumation of pre-formed pyritic grains: lines of evidence Fulreader (1957) and Kalliokoski (1966) argued that some or all of the pyrite in the Leicester lenses had secondarily replaced particles of originally different composition and that pyrite precipitation was essentially supergene. We agree that finely disseminated framboidal and drusy crustose pyrite between the large particles may have been precipitated after formation of Leicester lenses. Crustose pyrite commonly coats larger particles as well as grain contacts within grain-supported lens interiors; this indicates that such pyrite formed after deposition of lens debris. The formation of similar, early diagenetic, intergranular marcasite has been reported from the base of a turbiditic siltstone bed in the Mississippian Borden Formation (Maynard and Lauffenburger, 1978). We contend, however, that nodular, tubular, and steinkern grains were pyritized prior to exhumation from the Windom Member; these are pyritic allochems which were transported as sediment on the open sea floor. Reworking of Leicester pyrite grains, as implied by Park and Weiss (1972), is supported by several lines of evidence presented below. Firstly, pyritic burrow tube fragments, often conspicuously current-aligned on lens surfaces (Fig.5.C), are identical to those in the uppermost Windom Member and are believed to be derived from that unit through submarine

173

Fig.8. Reorientationof pyritic geopetal structures on top lodi discontinuity:schematicreconstruction. Note multiple attitudes of stalactites within exhumedgrains and protrusion of in-situ burrow tubes above discontinuity surface. exhumation. In-situ Windom tubes are pyrite-impregnated axes of shaft burrows and Zoophycos spreiten margins. Pyritic tubes of similar morphology, found in near-surface Holocene muds near Norway, have been ascribed to activity of near-surface burrowing polychaetes (Thomsen and Vorren, 1984); in this instance pyritization occurred in minimally aerobic surface muds characterized by local reducing microenvironments around burrows. The centers of many Windom and Leicester tubes are spar-filled voids surrounded by a rim or sheath of framboidal pyrite. Such pyrite linings are believed to have formed through bacterial sulfate reduction resulting in impregnation by pyrite of a stiff organic wall or pellet-rich zone surrounding the original burrow. Axial spar-filled voids in such tubes are particularly characteristic of a calcareous mudstone layer (Lodi Limestone Bed) below a younger Geneseeage discontinuity (Fig.7.B-E). Exhumation of these tubes is illustrated by protrusion of broken tube ends above the Lodi discontinuity, as well as their concentration in lenses (Figs.7.D and 8). Exhumation of nodular and tubular pyrite from the Windom is also supported by the lack of similar in-situ pyrite in the Geneseo. The Geneseo contains abundant fine-grained disseminated pyrite but generally lacks the nodular variety. Moreover, trace fossils are relatively scarce in the Geneseo and the sediment is typically laminar; a few Chondrites burrows are found at some levels in the black shale, but these are never pyritized. Recent studies indicate that most pyritic molds form within tens to hundreds of years of burial (Berner, 1970, 1981; see Dick, 1982; Hudson, 1982; Dick and Brett, 1986). The time interval between the Windom and Leicester is such that early diagenetic pyrite would have been present in Windom muds long prior to exhumation. Moreover, the anaerobic bottom setting indicated by Genesee sediments favored stability of exhumed pyrite on the sea floor.

174 There is additional physical evidence of pyrite exhumation. Many delicate Hamilton fossils were subjected to post-burial compaction and partial compression before pyrite formation, thus displaying deformation at the time of pyrite impregnation and chamber-filling. When subsequently exhumed such reinforced moldic fossils retain the flattened form and associated fractures even though they have been rolled and realigned. One compressionally flattened orthoconic cephalopod, found in the Leicester, is rotated normal to the original orientation in which it was flattened and mineralized (Fig.6.A, B). If a non-pyritized, but pre-flattened phragmocone had been exhumed it undoubtedly would have disintegrated; the fact that it was a deformed pyritic steinkern at the time of exhumation explains its coherence and orientation. In addition, we have observed reoriented geopetal structures in pyritic debris lenses. Early diagenetic pyritic stalactites are known to form on the roofs of pyrite-lined cavities within fossils (see Dick, 1982; Hudson, 1982, for detailed description and interpretation of such structures); stalactites always extend downward vertically during formation and stalagmites are never observed (Fig.7.A). Stalactitic pyrite is common within both in situ and reworked, pyrite-lined burrow tubes. In-situ hollow tubes below the Leicester and below an analogous, but younger reworked pyrite deposit associated with the Lodi discontinuity, always shows vertical stalactites (Fig.7.B), but tubes in pyritic lenses at both levels show stalactites in random orientations (Fig.7.E). Figure 8 is a schematic reconstruction of the exhumation process. Reoriented orthocone and Tornoceras steinkerns in the Leicester similarly show geopetal stalactites in various orientations; the above features unequivocally indicate reorientation of pre-formed pyrite during lens formation. Moreover, both in-situ Windom- and reworked Leicester Allanella molds have identical "meniscus" collars of coarsely crystalline "over-pyrite" (sensu Hudson, 1982) along the shell commissure (Fig.6.F). It seems highly unlikely that overpyrite growths identical to those on in-situ Windom brachiopods would have formed secondarily, within lenses, in precisely the same way. Finally, breakage patterns on Leicester grains also indicate that such particles were pyritic at time of transport. Reflected light examination of various clasts shows clear breakage of stiffly cemented tube material (Fig.7.F). Many of these breaks appear arcuate in cross-section view implying probable conchoidal fracture of hard crystalline pyrite.

Leicester deposition: fate of carbonate sediment A problematical feature of the Leicester is complete absence of calcareous shells and reworked Windom carbonate debris at most localities, particularly in view of the fact that underlying Windom concretion beds and coralbrachiopod zones are regionally truncated (Fig.9.A: Brett and Baird, 1982; Baird and Brett, 1983). Aside from local minor calcarenitic deposits capping the Tully Formation in the Seneca Lake Valley, the Leicester lacks calcareous material except for diagenetic sparry calcite cement between grains (Figs.5.E

175

Fig.9. Dissolution of reworked carbonate debris. A. Leicester pyritic remani~ material (a) resting on Bellona Coral Bed (b) of Tully Formation. Polished vertical section (Loc. 10). B. North Evans Limestone Member of Genesee Formation showing pelmatozoan encrinite and reworked limestone fragments (Loc. 2). C. Horizontal photomicrograph section showing Leicester grainstone with mixture of pyritic clasts and calcareous pelmatozoan ossicles (Loc. 3). D. Bedding plane view of North Evans remani~ material. Note pelmatozoan debris, brachiopods, and large ptyctodont crushing tooth (arrow) (Loc. 2). Bar for A, B and D is 1.0 cm; C is 0.5 cm.

and 6.G). It is p r o b a b l e t h a t some c a l c a r e o u s debris e x h u m e d d u r i n g postW i n d o m erosion h a d a l r e a d y b e e n d e s t r o y e d by a b r a s i o n a n d / o r dissolution by the time the L e i c e s t e r lenses h a d a c c u m u l a t e d . H i a t u s - c o n c r e t i o n s and large e x h u m e d fossils are, similarly, r a r e to a b s e n t a l o n g o t h e r d i s c o n t i n u i t i e s w h e r e erosive o v e r s t e p is indicated, such as a l o n g the bases of the M o s c o w and Tully f o r m a t i o n s (Heckel, 1973; Baird, 1979). H o w e v e r , t h e lack of calcitic shells and o v e r w h e l m i n g p r e p o n d e r a n c e of pyritic clasts implies a f u r t h e r m e c h a n i s m for the d e s t r u c t i o n of c a l c a r e o u s material. It appears t h a t the a n a e r o b i c to d y s a e r o b i c Genesee p a l e o e n v i r o n m e n t was the setting for significant s u b m a r i n e dissolution of r e w o r k e d c a l c a r e o u s debris. T h e L e i c e s t e r is t h u s believed to be a distilled chemical residue of f o r m e r l y more a b u n d a n t remani~ material. C a r b o n a t e dissolution in deep, cold, o c e a n i c w a t e r s is well d o c u m e n t e d , but this process is also i m p o r t a n t in shallow shelf and even lake settings (see Kindle, 1915; A l e x a n d e r s s o n , 1978). Significant c a r b o n a t e dissolution takes place in oxygen-deficient settings. In c e r t a i n o x y g e n m i n i m u m zones low pH c o n d i t i o n s are o b s e r v e d owing to high levels of dissolved CO 2 a s s o c i a t e d with o x y g e n d e p l e t i o n ( M c I l r e a t h and James, 1980); w h e r e such w a t e r overlies the

176 substrate, exposed carbonate grains are dissolved and the sediment becomes enriched with silica and organic matter. In order for extensive carbonate dissolution to occur, three main conditions should operate: firstly, low pH conditions or carbonate undersaturation must be present along the substrate; secondly, slow rates of sediment accumulation must exist to maximize skeletal exposure on the bottom; and thirdly, bottom current activity must occur so as to remove sediment cover which would buffer and protect calcareous material. Tanabe et al. (1984), in describing partial dissolution of aragonitic cephalopods in Liassic black shale, noted that the extent of dissolution of individual shells was a function of rapidity and completeness of shell burial. The argument for dissolution in a sedimentstarved setting is further suggested by the stratigraphically condensed character of Geneseo and analogous black shales. How can anaerobic conditions be reconciled with evidence for strong bottom currents? This may be less of a problem than initially perceived; firstly, the discrete character of Leicester lenses and the presence of silt laminae and Styliolina traction sheets, higher in the Geneseo, indicate that bottom current flow was episodic. Thus, between episodes of flow, quiet, stagnant periods could have ensued; it is at these times that carbonate dissolution most likely took place. The actual current scour and erosion transport events are believed to have been variably aerobic; the deep-water current processes discussed below do bring some oxygen to the substrate. However, the interbedded laminar shale intervals between lenses indicate that scour episodes were separated by long periods of relative stagnation. It is important to note here that exposed pyrite is able to survive brief periods of aeration. Recent chemical studies indicate that pyrite, once formed, may remain stable in a minimally oxygenated environment for considerable time (Reaves, 1984). Hence, the occurrence of pyrite allochems in hydraulic concentrations does not violate chemical models for pyrite stability, given that the prevailing bottom environment was predominantly anaerobic. The process of carbonate dissolution on and within sulfide-rich sediments is not fully understood. Van Straaten (1967) and Reaves (1984) argue persuasively that carbonate dissolves when adjacent pyrite or iron monosulfide is oxidized; repeated oxidation of the sulfide component releases H2SO 4, resulting in lowered pH and consequent dissolution of aragonite followed by calcite (Van Straaten, 1967). This concept is further supported by Sholkovitz (1973) who found exposed shells better preserved in the anaerobic Santa Barbara Basin floor than on the dysaerobic basin flank; unlike the oxidative (acidic) setting of the slope, the basin floor is distinctly alkaline, favoring carbonate preservation. This model does not explain features observed in certain ancient deposits. However, Seilacher et al. (1976), Hudson (1982) and Tanabe et al. (1984) have noted complete or nearly complete dissolution of exposed or minimally buried shells within laminar, black shales of Liassic age. Similarly, we note the

177 complete absence of calcareous remani~ material associated with the Leicester at almost all localities. Moreover, we have not observed iron hydroxides associated with the Leicester or analogous reworked pyrite deposits; the absence of oxidized pyrite suggests that current episodes were either brief and infrequent or that they introduced only minimally aerobic water. We believe that the intergrain environment in lenses may have been a particularly important area of dissolution. Comparison of thin vs. thick Leicester deposits reveals differences in amount of residual carbonate grains; lenses exceeding 1 cm in thickness generally lack carbonate allochems (Figs.4.C, 5.D and 6.G), but several very thin deposits yield local concentrations of calcareous crinoid ossicles and Windom brachiopods (Fig.9.C). Calcareous grains, where found, occur in patchy concentrations at the bases of lenses, around their periphery, and locally on their upper surfaces. Rarely, however, are clasts of this sort observed in lens interiors. At Cazenovia Creek (Loc. 3), crinoid ossicles and calcareous shells of the brachiopod Ambocoelia? praeumbona exhumed from the Amsdell Bed of the underlying Windom Member (Brett and Baird, 1982), are common on the top surfaces of Leicester lenses and along the unconformity surface between lenses, but they are absent from lens interiors. Apparently the highly concentrated condition of the pyrite grains may have produced an acidic microenvironment in lens interiors leading to dissolution of any remaining carbonate. If pyrite allochems are scarce relative to reworked calcareous material, the pyrite may be sufficiently dispersed in the mix so as to be adequately neutralized or buffered. This may explain local occurrences of calcareous debris lenses on the upper contact of the Tully Limestone, a unit containing only a minor amount of pyrite. CARBONATE-TO PYRITE GEOCHEMICALAND BATHYMETRIC SPECTRUM One way to study causal factors associated with pyrite lens formation is to examine coextensive carbonate deposits which grade basinward into the Leicester condition and to study analogous beds at other stratigraphic levels. Although the Genesee-Windom unconformity is truncated by the younger Genesee-North Evans discontinuity in central Erie County and is not observed west of this area (Figs.2 and 3.A), calcarenitic remani~ material on this younger discontinuity provides an important clue as to how Leicester would have appeared in shallower, oxygenated water. The North Evans is a thin, 2-18 cm (1-7 in.), blanket concentration of bone fragments, pelmatozoan debris, conodonts, and glauconite-coated hiatus-concretions (Fig.9.B,D) which typically marks the base of the Genundewa Limestone Member, a pelagic, deeper water carbonate unit (Hussakoff and Bryant, 1918; Brett, 1974; Brett and Baird, 1982). Oxygen concentrations at the bottom were quite low as suggested by a sparse benthos at this level. The North Evans apparently records submarine erosion during a minor regression event; transport and burial of exhumed carbonate took place under mildly dysaerobic conditions at most localities.

178

Unlike Leicester accumulations the North Evans is a continuous sheet deposit in most sections rather than a series of lenses. Differences between these two units reflect fundamental differences in paleobathymetry, water chemistry, and hydraulic properties of transported material during deposition. The latter two variables are most important in explaining the different depositional geometry from east to west. Firstly, dissolution of the carbonate component reduces the volume of reworked material, leaving only an insoluble residue fraction. This is particularly well illustrated by lateral change in physical character of the North Evans accumulation along the Lake Erie Shore (Loc. 1) where an unnamed black shale unit above the North Evans separates it from Genundewa carbonates; as the black shale appears, the North Evans grades southward from a continuous carbonate lag blanket to discontinuous thin lentils of remani~ composed almost entirely of pyritic grains and bone fragments over a distance of two kilometers. Secondly, the dense and coarse nature of Leicester pyritic clasts, should explain in part their discontinuous distribution on the unconformity surface. Although traction hydraulics of pyrite gravel are not well understood compared with other sediment types, it is not surprising that accumulations of such material probably would differ fundamentally from blankets of pelmatozoan calcarenite. This is particularly well illustrated by conspicuous sorting segregation of pyritic allochems in the sole of a thin bed of current-rippled fine siltstone (Fig.10) observed in shales of the lower Genesee Formation near Cayuga Lake (Loc. 15). In Erie County, the North Evans discontinuity oversteps the Leicester indicating that Leicester pyritic debris was secondarily reworked into the North Evans bottom setting (Brett and Baird, 1982). Although some reworked pyritic grains are observed in the North Evans, most Leicester pyrite debris probably was chemically unstable and destroyed in the more oxygenated North Evans environment at sections where the latter unit is directly overlain by the Genundewa Limestone.

Fig.10. Entrainment of reworked pyrite in siltstone bed. Current-generated tractional siltstone layer associated exhumed pyritic grains. Note density-shape-sorting segregation of pyritic debris from the overlying rippled silt (Loc. 15). Bar is 1.0 cm.

179

A

Dysaerobic Anaerobic

b Older bedrock '

units

Aerobic C

.a

Dysaerobic

Fig.11. Westward depositional onlap of Leicester and related remani~ deposits as .aey should appear in sections beneath Lake Erie. A. Depositional onlap of black and dark gray Genesee muds (b) onto the Taghanic Unconformity surface. Leicester lenses (a) on exposed slope and on leading edge of black mud deposit. B. Regression-induced submarine erosion of Genesee and pre-Genesee deposits to west; deposition of Styliolina-rich Genundewa Limestone Member (d) in east. North Evans remani~ material (c) accumulates in dysaerobic to minimally aerobic setting. C. Renewed transgression and return to anaerobic conditions. Black mud [West River Shale Member; unit (e)] buries North Evans-Genundewa carbonates during continued westward onlap. Leicester-type deposits (f) redevelop along anaerobic Taghanic discontinuity surface under conditions identical to those for Lower Genesee deposition. Given the fact t h a t Geneseo a n d y o u n g e r beds lap o n t o the eroded s u r f a c e west of Erie C o u n t y , it is possible to e x t r a p o l a t e as a p r e d i c t i v e model the type of debris w h i c h s h o u l d o c c u r o n the T a g h a n i c U n c o n f o r m i t y f u r t h e r west. Since t h e N o r t h E v a n s a n d o v e r l y i n g G e n u n d e w a m e m b e r s are j u x t a p o s e d directly on the W i n d o m in Erie C o u n t y , one w o u l d expect these to be the first u n i t s to d i s a p p e a r by o n l a p west of t h e r e (Fig.ll.B). Once these beds lap o n t o the u n c o n f o r m i t y , b l a c k a n d d a r k g r a y shales of the o v e r l y i n g West R i v e r

180 Member (upper Genesee Formation) would be juxtaposed on the Hamilton. The return to black shale conditions should result in reappearance of starved Leicester-type pyritic lenses on the contact, albeit of distinctly younger age than any Leicester so far observed, given a ready source of erodable subjacent pyrite (Fig.ll.C). Although the North Evans erosion event is medial Genesee in age, it provides an indirect glimpse of what the aerobic, upslope parts of the Taghanic erosion surface must have looked like during much of Late Devonian time. Since onlap of the erosion surface occurred only in the deeper water regime (see Taghanic paleoslope erosion model below), midslope "North Evans" and shallower calcareous lag deposits would be rarely, if ever, preserved. Only major widespread regression events followed by transgression and continued onlap (Fig.ll) would have left a downdraped mantle of shelf-margin relict carbonate sediment within the basin sequence proper. MECHANISMS FOR TRANSPORTAND DEPOSITION OF PYRITE CLASTS Black shales have traditionally been viewed as products of deposition in quiet, stagnant settings. However, Geneseo deposits show abundant evidence of current activity, best exemplified by the reworked pyrite. Additional evidence of current flow such as entrainment of silt in traction ripples (Fig.10), plus alignment of cephalopod fragments and Styliolina shells, is observed in numerous Devonian back shale units. Evidence of episodic current activity is likewise reported from Mesozoic black shales (see Brenner and Seilacher, 1978; Kauffman, 1981). The Hjiilstrom diagram predicts that strong currents are required for erosion of fine-grained sediment. The association of pyrite allochem gravel with mud-floored discontinuities both at the base of, and higher in the Genesee Formation, would thus appear to support an argument for strong current flow coincident with black mud deposition. Furthermore, presence of mud rip-up clasts in Leicester lenses indicates that underlying deposits were generally consolidated at the time of scour events. A simplistic argument for extreme flow velocities, however, must be tempered with the fact that pyritic grains, once exhumed, would have enhanced the erosion process due to abrasive rolling and saltation. This feedback effect is suggested by the aforementioned scour grooves on the bases of lenses (Fig.5.A, E). Normal wave action is not believed to have had much effect on the Geneseo substrate owing to its depth, but major storm events may have generated strong currents along the basin margin slope. Major storm waves are known to periodically erode the sea floor and transport sandy sediment at depths of up to 200 m (Liebau, 1980). Moreover, storms are sometimes important in generating boundary currents below storm wave base; erosive storm-generated bottom currents are known to produce scour troughs and furrows 200 m below the surface of Lake Superior (Johnson et al., 1984). Additionally, warm surface currents such as the Gulf Stream are known to generate latent, cyclone-like

181 gyres of strong (>/60 cm/s) current flow which descend to great depth and which dissipate slowly (Hollister et al., 1984). Considerable coarse sediment is entrained by these strong currents as the disturbance passes. Hollister et al. (1984) believe that such currents are a major cause of seabed scour and sediment transport in bathyal and deeper settings. The discrete and multiplystacked character of many Leicester lenses indicates that current flow was distinctly episodic with long intervening quiet periods (Figs.4.D and 6.G); this irregular pattern is consistent with that expected for major storm events. It is also probable that currents and turbulence, generated within the pycnocline, are, at the least, an accessory agent in the erosion process. In modern seas, storms and other processes generate internal waves which transfer energy within the water column (Emery, 1956; Lafond, 1962). In particular, these waves are known to shoal against sloped substrates where the boundary layer intercepts the bottom; this impingement is known to generate turbulence, both experimentally (Southard and Cacchione, 1972), and in modern marine settings, where it is believed to be important in transporting shelf sediments (Karl et al., 1983). In a recent study of regressive, prodeltaic facies of the Genesee Formation, Woodrow (1985) suggested that the base of the pycnocline was marked by internal waves which broke against the prodelta slope, producing a distinctive interval of shell-rich, wave- and current-winnowed sandstones; he argues that the observed upward change from turbiditic slope facies into this sandstone interval marks a "process boundary", separating quiet, basinal waters below the pycnocline from wave-influenced layers within and above it. TAGHANIC PALEOSLOPE EROSION MODEL We believe that the impingement of internal waves on the sediment-starved Taghanic basin margin slope would have produced effects markedly different from those noted by Woodrow (1985); such a process occurring in this setting may explain both the association of Leicester remani~ with basal Geneseo black muds and the seemingly paradoxical juxtaposition of black shale facies over a widespread unconformity. This postulated mechanism of submarine erosion can be outlined as follows. Marine transgression would presumably have been timed with ascent of the pycnocline such that the level of impinging internal waves would have migrated upslope with deepening. In the absence of significant sediment supply, this turbulence would have eroded the bottom, leaving coarse debris as lag accumulations. Over a period of time, the impinging boundary layer would have migrated upslope to some maximally transgressive position, leaving in its wake, a continuous erosion surface that would be overlain by overlapping black mud, recording deposition below the pycnocline. Leicester and analogous pyritic remani~ deposits would thus record waning current flow timed with the onset of black mud deposition and should occur shingled within the basal part of the black shale sequence. On balance, we believe that deep storm turbulence can explain many

182 current-produced features observed in the Geneseo, but we feel that dynamic boundary layer effects, associated with basin stratification, may explain more fully the distinctive character of the Taghanic discontinuity as ~vell as numerous similar discontinuities flooring black shale deposits elsewhere in the Devonian. The main events following Tully carbonate deposition were major marine transgression and widespread submarine erosion of the Tully and older units followed by deposition of black mud. We believe that the basin trough had a NE-SW transverse profile similar to that figured in Broadhead et al. (1982, p. 19). Reduced sediment supply to the southeastward sloping (cratonward) basin flank during transgression, resulted in sediment starvation and erosion along the basin margin over a long time span (see nearshore sediment alluviation models of McCave, 1967; Johnson and Friedman, 1969). It is clear from the evidence presented that strong bottom currents, or deep-seated storm agitation episodically scoured the mud-floored Taghanic slope; this apparently also prevented net sediment accumulation until burial of the discontinuity by subsidence-related sedimentary onlap. Given the evidence of significant current activity in basal Geneseo deposits, it seems peculiar that black, laminated muds should be the first sediment type to bury the erosion surface and associated pyrite; would not the fine pelagic fraction get winnowed away? As shown from field studies, currents obviously did not remove the entire black mud interval between double- or multiply-stacked lenses (Figs.4.D, 5.D and 6.G). Thus, we believe that the limit of black mud deposition at any given time during onlap probably oscillated, moving slowly shelfward during quiet periods and abruptly basinward during pulses of bottom erosion with a net slow westward drift through time (Figs.ll and 12.A). By direct analogy, Flood (1981) noted that substrates affected by episodic furrow scouring by currents can be characterized by net sediment accumulation if longer intervening quiet periods occur between current events. Slowly deposited pelitic sediment is low in water content and is typically cohesive; this is particularly indicated by Geneseo rip-up clasts in Leicester lenses and aforementioned lack of load structures under lenses. Such evidence shows that firm basinal muds onlapped the discontinuity below some critical energy threshold, these being sufficiently firm to retain their position basinward of the mud line. The absence of pyrite lenses more than 10 cm above the discontinuity in all outcrops is apparently a function of the maximal lateral (basinward) distance that pyrite could be transported past the shelfward limit of black mud (Fig.12.A). The Taghanic erosional surface presumably shoaled to the basin margin; a shallow shelf or shore regime must have been present to the north and west of Buffalo during Genesee time (Fig.12.B). In this shallow, moderate- to highenergy setting, biogenic carbonate would have accumulated in areas of low clastic influx. Currents in such a setting would have produced a calcareous lag blanket, perhaps similar to comminuted pelmatozoan deposits of the North Evans.

183

~'~!ii~

~

,'-~'i ".

'

Fig.12. Downslope transport of Leicester debris: schematic reconstruction. A. Seascape during Leicester deposition; pyrite "lenses" (black) are shown as flute-shaped accumulations in furrows or similar runnel-like channels aligned parallel to current flow. Dotted lines paralleling flutes are scour grooves (micro furrows?) cut into underlying Windom muds. Leicester lenses migrate over marginward limit of onlapping black muds; and Leicester lenses are imbricated (shingled) in basal Geneseo deposits. Units include: a = tractional Leicester deposits on discontinuity surface; b = Windom key beds bevelled and overstepped by Taghanic Unconformity; c = Geneseo black mud deposits; d = shingled Leicester lenses in the Geneseo. Inset; e = shows actual size of lenses relative to scale for larger block. B. Downslope dissolution of remani~ material; basinward transition from calcareous palimpsest in shallow shelf regime to pyrite and bone residue in basin with inferred accompanying change from wave-induced transport to bottom current-induced traction. Units include: a=shallow-water carbonate shelf; b=distal shelf calcarenite blanket; c = sediment-starved, current-swept upper slope with thin or discontinuous remani~ cover; d=mid-slope region with starved-lenses of mixed carbonate and pyrite allochems; e = l o w e r slope and basin with Leicester starved-lenses partly overlapping Genesee black muds; f = Hamilton key beds beveled and overstepped by Taghanic Unconformity.

Storm- or tide-generated currents would have swept some of this debris basinward; this particle transport would have acted to abrade the substrate freeing additional bioclasts and pyrite. In shallower depths mechanical abrasion and oxidation would have removed the pyrite from circulation, but in the vicinity of Genesee mud accumulation, the pyrite would have been more chemically stable. Anaerobic bottom conditions, prolonged bottom exposure, and hydraulic commingling of the pyrite w~th carbonate lag debris would then have set the stage for carbonate dissolution, as discussed earlier (Fig.12.B). Leicester lenses reflect the final configuration of transported, reworked debris that was moved by bottom currents on the submarine slope. Stabilization and burial of these lenses would have occurred at, or immediately basinward of, the upslope limit of black mud deposition at any given time

184 (Fig.12.A). It is important to reemphasize that some Leicester remani6 material was moved by traction past this upslope mud line such that many Leicester lenses ultimately came to rest over basal Geneseo muds; pyritic debris thus came to be shingled with black shale along the diachronous base of that unit (Figs.2, 3.A and 12.A). An intriguing question relates to the fate of upslope calcareous remani6 material and shelf carbonates that were coeval to Genesee deposits; would not this basin fill up with progressively shallower deposits through time, and would there not be a westward gradation from Leicester-type debris to calcareous remani6 material as water depth gradually decreased? Indeed, as there was an eastward-dipping slope in western New York, there presumably would have been shallow shelf carbonates equivalent to the Leicester farther to the west in Ontario and Ohio. Such deposits would only be preserved if a tectonically immobile basin had been filled gradually with sediment. As observed, black shale of medial Frasnian age overlies the Taghanic unconformity to the west of New York State suggesting that the basin axis was transient with time through the late Devonian. Furthermore, several workers believe that the basin's west flank had migrated progressively westward through downward flexing as a result of tectonic adjustment and sediment loading (see Dennison, 1983; Rollins et al., 1984; Ettensohn, 1985, for dynamic basin model). What would have been the fate of calcareous, shallower shelf and slope deposits contemporaneous to Leicester in Ontario or Ohio? If the above flexural models are correct, such deposits would no longer be present in the rock record due to stratinomic effects of westward basin drift; in any given locus the sediment-starved, upslope basin margin would have progressively subsided such that mobile carbonates would have changed through time and bathymetric descent to pyrite and bone residue before being buried by onlapping black muds. Thus, the shelf margin and upslope erosional settings, originally present, would have been erased from the rock record. However, the upslope environment is recorded indirectly through subsidiary events such as the North Evans regression, which left upslope-type calcareous remani6 deposits downdraped and isolated within the basin proper (Fig.ll.C). RECURRENCE OF REWORKEDPYRITE IN THE GEOLOGICRECORD Reworked pyrite has been described only rarely in the rock record; it is known from certain sedimentary ore deposits such as the Precambrian Witwatersrand (Simpson and Bowles, 1977). Love (1971) described reworked pyrite occurring as a current-sorted fraction of reworked, sand-sized polyframboids within turbidites in Welsh Silurian deposits. Keith and Friedman (1977) mention that resedimented pyrite is observed locally in distal turbidites or contourites within the Ordovician Taconic sequence of New York. It is also possible that thin layers of pyritic ooids, associated with the Ordovician Wabana oolitic hematite deposit in Newfoundland may be detrital (see Hayes,

185

Fig.13. Reworked pyrite at Frasnian-Famennian contact. Reworked pyritized burrow tubes at one of several discontinuities at and near base of Dunkirk Member (black shale deposit). A. Conspicuously-aligned burrow tubes at base of Dunkirk. Rythus Creek in Eden, Erie County. B. Lower surface of remani~ debris lentil on discontinuity. Note ~jackstraw"-like tube concentration and small size of tubes. Walnut Creek below N.Y. State Thruway overpass bridge, Chautauqua County. Bar for A and B is 1.0 cm. 1915), although a subsequent worker (Kalliokoski, 1966) argues t hat most, if not all, of this pyrite had replaced non-pyritic grains following their deposition. The present aut hor s have found Devonian pyritic allochems at no less t h a n eight stratigraphic levels in addition to t h a t of the Leicester. At the base of the Penn Yan Shale Member, reworked pyritic tubes and nodules occur in starved t r a ctio n lenses along a localized diastem at the level of the Lodi Limestone Bed west of Seneca Lake (Locs. 12 and 13). These lenses, containing abundant hollow, pyritic, burrow tubes, moldic goniatites, and bone debris, have a striking resemblance to those of the Leicester. At least two other horizons in the Genesee F o r m at i on exhibit similar features. Resedimented pyrite also occurs above the Genesee Formation. Preliminary studies show the presence of reworked pyritic grains both at, and immediately above, the base of the Dunkirk Shale Member (basal Famennian) in Chautauqua and Erie Counties in westernmost New York. The pyritic grains are predominantly molds of minute (0.2-0.5 mm diameter) straight burrows which may have been produced by nematodes or minute polychaetes. Identical pyritic burrows occur in-situ in gray muds below the black, laminar D unki rk facies. These jackstraw-like grains occur reworked in lenses or as sheet accumulations in the basal centimeter of black shale (Fig.13) and are frequently strongly current-aligned (Fig.13.B). Along a discontinuity separating the Chagrin Shale Member and overlying Cleveland Shale Member (medial Famennian) in n o r t h e a s t e r n Ohio is a lenticular, bone-rich lag deposit composed almost entirely of pyritic clasts similar to those in many Leicester sections (Mausser, 1982). At Skinner Run in Cleveland, Ohio, it marks the top of a gray, bioturbated claystone sequence (Chagrin) yielding imsitu nodular pyrite, and the base of the hard, black

186

Cleveland facies which is analogous to the basal Geneseo. Mausser (1982, p. 82-84) and the present authors believe that Skinner Run clasts are exhumed early diagenetic pyrite. Deposits containing variable admixtures of carbonate and pyritic allochems are known from a discontinuity in the Ludlowville Formation of the Hamilton Group. The Hamilton occurrence is associated with a mud-floored submarine discontinuity between pyrite-rich, burrowed shales of the uppermost Center-field Member and overlying dark gray shale of the Ledyard Member. Unlike pyrite occurrences previous discussed, this pyrite has been disturbed and moved by organisms. Tubular burrow pyrite typical of the underlying shale occurs broken and jumbled together with shells and prefossilized conularid steinkerns along the discontinuity. Although the pyritic tubes, shells, and steinkerns were exhumed during post-Centerfield erosion, continued aerobic bottom conditions allowed deposit-feeding organisms to colonize and burrow through the soft, erosion surface; burrowers commingled existing surficial remani~ material and may have acted to dislodge and convect up additional pyritic tubes from the underlying sediment. This is an example of quasiresedimented pyrite that occurs on a vertically churned, stratomictic erosion surface (sensu Baird, 1981). Our findings indicate that transported pyrite grains are by no means rare in the marine sedimentary record. Similar rock sequences composed of alternating oxic and anoxic argillaceous sediments should yield pyritic allochems as a variable component of remani~ material given that conditions suitable for formation of near-surface, early diagenetic pyrite as nodules, tubes, or steinkerns in mud develop periodically. Subsequent exhumation of this pyrite must occur in an anaerobic or minimally aerobic setting in order for it to remain chemically stable. Facies most likely to yield this kind of remani~ should be offshore, clay-tosilt grade, terrigenous sediment such as would be found in certain basin and basin margin sequences. Perhaps the most conducive setting for dccumulation of pyritic allochems would be sediment-starved muddy shelves of submarine slopes exposed to shifting water mass boundaries during transgression events; such conditions could produce erosion effects timed with onset of anaerobic bottom conditions, particularly in sloped areas. Such a predictive model appears to work for the Devonian, but it should also apply to comparable sequences within other time periods. However, the central test would be to predict occurrences of pyritic gravel on the modern sea floor. Thus far, reworked pyrite has only rarely been detected in the Phanerozoic (see Keith and Friedman, 1977) and not at all in modern seas; it appears that oceanic plains, rises, and slopes, may be poor analogs for the Devonian basin in that the substrate is usually variably aerobic (Hallam, 1981). We believe that partially restricted marginal basin seas are more likely to yield deposits such as those discussed herein. Van Straaten (1967) describes pyritic tubes in oxygen-poor, near-surface muds of Adriatic slope and basin settings; the occurrence of these tubes in such a setting along slope suggests

187 the potential for pyrite exhumation in some of the small modern basinal seas due to one or more current mechanisms discussed. Using the reverse axiom "the past is the key to the present" one should anticipate discovery of reworked pyrite on the modern sea floor. CONCLUSIONS The Leicester Pyrite Member of the Genesee Formation comprises laterally discontinuous lenses of reworked granule-to-gravel-sized pyritic clasts which occur along or immediately above the Taghanic unconformity across most of western New York. Although most Leicester lenses occur on the HamiltonGenesee disconformity, many occur above it within laminar black shale deposits of the basal Geneseo Member, indicating that Leicester deposition occurred during Genesee time under predominantly anaerobic conditions. This stratigraphic pattern agrees with biostratigraphic data which show that lenses become zonally younger westward, ascending through at least two conodont zones. Leicester grains, though densely pyritic, show evidence of current transport, particularly revealed by grain alignment and mechanical breakage of particles. Grains include rounded nodules, tubular rods with spar-filled, hollow cores, pre-pyritized fossil molds, and fish bones. Some, but not all, of this material is derived from erosion of underlying fossil-rich muds of the uppermost Hamilton Group; tubular grains have their source in pyritic burrow linings within the underlying gray mudstone and many fossil molds are of species characteristic of upper Windom muds beneath the disconformity. Several lines of evidence show that some Hamilton debris was partly to wholly pyritic at the time of exhumation. Leicester lenses generally lack carbonate except as intergranular cement; calcareous shells and reworked diagenetic carbonate are essentially absent except for rare bioclasts near or on lens surfaces. Calcareous debris was apparently subjected to preburial dissolution in a predominantly anaerobic setting, some of this occurring in the intergranular lens microenvironment. Leicester remani~ material is believed to have migrated downslope as starved-sediment accumulations along a gentle, southeastward-dipping paleoslope; this relict debris probably moved within shallow erosional furrows on a compacted mud surface. Upslope Leicester deposits no longer retained in the geologic record were probably expressed as thin traction sheets and stormwave tempestite accumulations of carbonate-rich relict sediment on a sediment-starved shelf. As this material migrated into downslope dysaerobic and anaerobic water it changed into a chemically distilled residue of pyrite allochems and bones, moved by very deep storm waves or bottom currents. The basinward transport limit of lens migration was just below the upslope depositional limit of Geneseo black mud deposition at any given time. The diachronous character of Taghanic remani~ material indicates westward burial of the discontinuity surface by onlap. The temporal duration of

188 this onlap across western New York and Ontario is on the order of several million years; juxtaposition of basinal deposits on the Taghanic Unconformity may reflect tectonic westward shifting of the basin axis while the basin was being filled; subsidence of the western basin flank may have resulted in the transformation of a sediment-starved carbonate shelf setting to a basinal erosional environment before final burial by black muds. Resedimented pyrite is observed along discontinuities below other black shale deposits. Reworked pyrite is observed along a diastem associated with the Lodi Limestone Bed higher in the Genesee Formation. Exhumed pyritic burrow tubes are abundant along the base of the Dunkirk Shale Member (basal Famennian) in western New York. Similar remani6 material occurs locally along the base of the medial Famennian Cleveland Shale in Ohio. In the underlying Hamilton Group, pyritic burrow fragments were vertically mixed by infaunal organisms, indicating that pyrite can survive exposure to bioturbation effects and minimal oxidation within sediment. Reworked pyrite is believed to occur widely through the Phanerozoic where currents eroded muddy shelves or slopes in anaerobic to minimally dysaerobic settings. Occurrence of pyritic burrow tubes in Pleistocene and modern marine muds suggests that present-day occurrences of pyritic lag sediment await discovery in certain marginal sea basins. ACKNOWLEDGMENTS John D. Hudson of the University of Leicester, Philip H. Heckel of the University of Iowa, Iowa City, and Ed Landing of the New York State Museum, Albany reviewed copies of the manuscript and provided useful criticism during its preparation. John Hudson also provided advice and encouragement during early phases of this investigation. Arthur Earith and Walter Meyer prepared polished sections slabs and thin sections; James Eckert and Stephen Speyer helped in preparation of figures; Margrit Gardner deserves special credit for her patience and diligence in typing the various Copies of this manuscript during its preparation. The project was partially funded by a grant from the donors to the Petroleum Research Fund of the American Chemical Society, and by NSF grant EAR 8313103. APPENDIX I -- Localityregister

Locality 1: Cliff exposures along Lake Erie shore between Eighteenmile Creek and Pike Creek in Town of Evans, Erie County (Angola and Eden 7.5' Quads.). North Evans Member exposed at base of Genesee Formation. Locality 2". Abandoned shale pit north of Big Tree Road, 1.2 mi (1.9 km) east of Bay View, Erie County (Buffalo S.E. 7.5' Quad.). Type section of Penn Dixie Pyrite Bed in Windom Member. North Evans Member at base of Genesee Formation. Locality 3". Cazenovia Creek, 0.3-0.8 mi (0.5-1.3 km) upstream from Northrup

189

Road bridge in Town of Elma, Erie County (Orchard Park 7.5' Quad.). Leicester Pyrite at base of Genesee Formation. Locality 4: Durkee Creek, 1.7 mi (2.7 km) southeast of Alden, Erie County (Corfu 7.5' Quad.). Thickest Leicester lens. Locality 5: Bowen Brook, 1.7 mi (2.7 kin) northwest of Alexander, Genesee County (Alexander 7.5' Quad.). Leicester Pyrite present. Windom Member half truncated below Genesee Formation. Locality 6". Taunton Gully, 1.8 mi (2.8 km) north of Leicester, Livingston County (Leicester 7.5' Quad.). Leicester Pyrite. Locality 7". Unnamed Creek 1.3mi (2.1 km) southeast of Frost Hollow, Ontario County (Hemlock 7.5' Quad.). Leicester Pyrite. Locality 8: Menteth Gully, 0.8 mi (1.3 km) south of Cheshire, Ontario County (Canandaigua Lake 7.5' Quad.). Leicester present. Locality 9: Gage Gully, 1.1 mi (1.8 km) south of Cottage City, Ontario County (Canandaigua Lake 7.5' Quad.). Leicester Pyrite, Tully erosional outlier (lens) also present. Locality 10: Flint Creek at Gorham, Ontario County (Rushville 7.5' Quad.). Easternmost Leicester occurrence; pyrite rests on upper surface of Tully Limestone. Locality 11: Kashong Glen at Bellona, Yates Country (Stanley 7.5' Quad.). Calcareous remani~ material at Tully-Geneseo unconformity. Locality 12: Unnamed ravine at Ross Point, 1.6 mi (2.6 km) northeast of Himrod, Yates County (Dundee 7.5' Quad.). Reworked pyrite at horizon of Lodi Limestone Bed, Genesee Formation. Locality 13: Plum Point Ravine, 0.9 mi (1.4 km) northeast of Himrod, Yates County (Dundee 7.5' Quad.). Reworked pyrite at horizon of Lodi Limestone. Locality 14: Sheldrake Creek, 2 mi (3.2 km) north of Interlaken, Seneca County (Sheldrake 7.5' Quad.). Westernmost exposure of Fillmore Glen beds. Locality 15: Unnamed ravine at Bergen Beach Point, 2 mi (3.2 km) east of Interlaken, Seneca County (Trumansburg 7.5' Quad.). Reworked pyrite in base of thin (1.20 cm) siltstone bed below Lodi Limestone. Locality 16". Willow Point Ravine, 1.9 mi (3 km) southeast of Taughannock State Park, Tompkins County (Ludlowville 7.5' Quad.). Fillmore Glen interval complete. REFERENCES Allen, J. R. L. and Friend, P. F., 1968. Deposition of the Catskill Facies, Appalachian Region, with notes on some of the Old Red Sandstone basins. In: G. deV. Klein (Editor), Late Paleozoic and Mesozoic Continental Sedimentation, N o r t h e a s t e r n North America. Geol. Soc. Am. Spec. Pap., 106: 21-74. Alexandersson, E. T., 1978. Destructive diagenesis of carbonate sediments in the eastern Skagerrak, N o r t h Sea. Geology, 6: 324-327. Baird, G. C., 1979. Sedimentary relationships of Portland Point and associated Middle Devonian rocks in central and western New York. Bull. N.Y. State Mus., 433: 1-23. Baird, G. C., 1981. Submarine erosion on a gentle paleoslope: a study of two discontinuities in the New York Devonian. Lethaia, 14: 105-122.

190 Baird, G. C. and Brett, C. E., 1983. Regional variation and paleontology of two coral beds in the Middle Devonian Hamilton Group of western New York. J. Paleontol., 57: 417-446. Baird, G. C. and Brett, C. E., 1985. Reworking of sedimentary pyrite on euxinic seafloors, Devonian, western New York. Bull. Am. Assoc. Pet. Geol., 69:236 (Abstr.). Berner, R. H., 1970. Sedimentary pyrite formation. Am. J. Sci., 268: 1-23. Berner, R. H., 1981. A new geochemical classification of sedimentary environments. J. Sediment. Petrol., 51: 359-365. Bowen, Z. P., Rhoads, D. C. and McAlester, A. L., 1974. Marine benthic communities of the Upper Devonian of New York. Lethaia, 7: 93-120. Brenner, K. and Seilacher, A., 1978. New aspects of the Toarcian Posidonia Shales. Neues Jahrb. Geol. Pal~iontol. Abh, 157: 11-18. Brett, C. E., 1974. Biostratigraphy and paleoecology of the Windom Shale Member (Moscow Formation) in Erie County, New York. In: 46th Annu. Meet. Guidebook, N.Y. State Geol. Assoc., pp. G1-G15. Brett, C. E. and Baird, G. C., 1982. Upper Moscow-Genesee stratigraphic relationships in western New York: evidence for erosive beveling in the Late Middle Devonian. In: 54th Annu. Meet. Guidebook, N.Y. State Geol. Assoc., pp. 19-63. Broadhead, R. F., Kepferle, R. C. and Potter, P. E., 1982. Stratigraphic and sedimentological controls of gas in shale-example from Upper Devonian of northern Ohio. Bull. Am. Assoc. Pet. Geol., 66: 10-27. Byers, C. W., 1977. Biofacies patterns in euxinic basins; a general model. In: H. E. Cook et al. (Editors), Deep-water Carbonate Environments. Spec. Publ. Soc. Econ. Paleontol. Mineral., 25: 5-17. Cluff, R. M., 1980. Paleoenvironment of the New Albany Shale Group (Devonian-Mississippian) of Illinois. J. Sediment. Petrol., 50: 767-780. Conant, L. C. and Swanson, V. E., 1961. Chattanooga Shale and related rocks of central Tennessee and nearby areas. U.S. Geol. Surv. Prof. Pap., 357, 91 pp. Conkin, J. E., Conkin, B. M. and Lipchinsky, L. Z., 1980. Devonian black shale in the eastern United States; Part 1. Southern Indiana, Kentucky, northern and eastern highland rim of Tennessee, and central Ohio. Univ. Louisville Stud. Paleontol. Stratigr., 12, 63 pp. Cooper, G. A., 1930. Stratigraphy of the Hamilton Group of New York. Am. J. Sci. 5th ser., 19: 116-134; 214-236. Cooper, G. A., 1957. Paleoecology of the Middle Devonian of eastern and central United States. In: H. S. Ladd (Editor), Treatise on Marine Ecology and Paleoecology. Geol. Soc. Am. Mere., 67: 249-278. Cooper, G. A. and Williams, J. S., 1935. Tully Formation of New York. Bull. Geol. Soc. Am., 46: 781-868. Dennison, J. M., 1983. Rowboat depositional model for Appalachian Basin. Abstr. Program Geol. Soc. Am., 15: 114. Dennison, J. M. and Head, J. W., 1975. Sea-level variations interpreted from the Appalachian Basin Silurian and Devonian. Am. J. Sci., 257: 1089-1120. Dewey, J. F. and Burke, K., 1974. Hot spots and continental break-up: implications for collisional orogeny. Geology, 2: 57-60. Dewey, J. F. and Kidd, W. S. F., 1974. Continental collisions in the Appalachian-Caledonian orogenic belt: variations related to complete and incomplete suturing. Geology, 2: 543-546. DeWitt, W. and Colton, G. W., 1978. Physical stratigraphy of the Genesee Formation (Devonian) in western and central New York. U.S. Geol. Surv. Prof. Pap., 1032-A, 22 pp. Dick, V. B., 1982. Taphonomy and depositional environments of Middle Devonian (Hamilton Group) pyritic fossil beds in western New York. Thesis. Univ. Rochester, 118 pp. (Unpublished). Dick, V. B. and Brett, C. E., 1986. Petrology, taphonomy and sedimentary environments of pyritic fossil beds from the Hamilton Group (Middle Devonian) of western New York. In: C. E. Brett (Editor), Dynamic Stratigraphy and Depositional Environments of the Hamilton Group (Middle Devonian) in New York State, 1. Bull. N.Y. State Museum, 457: 102-128.

191 Emery, K. O., 1956. Deep standing internal waves in California basins. Limnol. Oceanogr., 1: 35-41. Emery, K. O., 1968. Relict sediments on continental shelves of the world. Bull. Am. Assoc. Pet. Geol., 52: 445-464. Ettensohn, F. R., 1985. The Catskill Delta complex and the Acadian Orogeny: A model. In: D. L. Woodrow and W. D. Sevon (Editors), The Catskill Delta. Geol. Soc. Am. Spec. Pap., 201: 39-50. Ettensohn, F. R. and Elam, T. D., 1985. Defining the nature and location of a Late Devonian-Early Mississippian pycnocline in eastern Kentucky. Geol. Soc. Am. Bull., 96: 1313-1321. Flood, R. D., 1981. Distribution, morphology, and origin of sedimentary furrows in cohesive sediments, Southampton water. Sedimentology, 28: 511-529. Flood, R. D., 1983. Classification of sedimentary furrows and a model for furrow initiation and evolution. Geol. Soc. Am. Bull., 94: 630-639. Friedman, G. M. and Johnson, K. G., 1968. The Devonian Catskill deltaic complex of New York: type example of a tectonic delta complex. In: M. L. Shirley and J. A. Ragdale (Editors), Deltas in their Geologic Framework. Houston Geol. Soc., Houston, Tex., pp.189-211. Fulreader, R., 1957. Geologic Studies of the Leicester Pyrite. Thesis. Univ. Rochester, 112 pp. (Unpublished). Hallam, A., 1981. Facies Interpretation and the Stratigraphic Record. Freeman, San Francisco, Calif., 291 pp. Hayes, A. O., 1915. Wabana iron ore of Newfoundland. Geol. Surv. Can. Mem., 78, 163 pp. Heckel, P. H., 1973. Nature, origin and significance of the Tully Limestone. Geol. Soc. Am. Spec. Pap., 138, 244 pp. Hollister, C. D. and Heezen, B. C., 1972. Geological effects of ocean bottom currents. In: A. L. Gordon (Editor), Studies in Physical Oceanography, Vol. 2. Gordon and Breach, New York, N.Y. pp. 37-66. Hollister, C. D., Nowell, R. M. and Jumars, P. A., 1984. The dynamic abyss. Sci. Am., 250: 42-53. Huddle, J. W., 1981. Conodonts from the Genesee Formation in western New York. U.S. Geol. Surv. Prof. Pap., 1032-B, 66 pp. Hudson, J. D., 1982. Pyrite in ammonite-bearing shales from the Jurassic of England and Germany. Sedimentology, 27: 639-667. Hussakoff, L. and Bryant, W. L., i918. Catalog of the fossil fishes in the Museum of the Buffalo Society of Natural Sciences. Bull. Buffalo Soc. Nat. Sci., 12: 1-198. Johnson, J. G., 1970. Taghanic onlap and the end of North American Devonian provinciality. Bull. Geol. Soc. Am., 81: 2077-2106. Johnson, K. G. and Friedman, G. M., 1969. The Tully clastic correlatives (Upper Devonian) of New York State: a model for recognition of alluvial dune?, tidal, nearshore (bar and lagoon), and offshore sedimentary environments in a tectonic delta complex. J. Sediment. Petrol., 39: 451-485. Johnson, T. C., Halfman, J. D., Busch, W. H. and Flood, R. D., 1984. Effects of bottom currents and fish on sedimentation in a deep-water, lacustrine environment. Geol. Soc. Am. Bull., 95: 1425-1436. Kalliokoski, J., 1966. Diagenetic pyritization in three sedimentary rocks. Econ. Geol., 61: 872-885. Karl, H. A., Carlson, P. R. and Cacchione, D. A., 1983. Factors that influence sediment transport at the shelfbreak. In: D. J. Stanley and G. T. Moore (Editors), The Shelfbreak: Critical Interface on Continental Margins. Spec. Publ. Soc. Econ. Paleontol. Mineral., 33: 219-231. Kauffman, E. G., 1981. Ecological reappraisal of the German Posidonien-Schiefer (Toarcian) and the stagnant basin model. In: J. Gray, A. J. Boucot and W. B. N. Berry (Editors), Communities of the Past, Hutchinson and Ross, Stroudsburg, Pa., pp. 311-381. Keith, B. D. and Friedman, G. M., 1977. A slope-fan-basin-plain model, Taconic sequence, New York and Vermont. J. Sediment. Petrol., 47: 1200-1241. Kindle, E. M., 1915. Limestone solution on the bottom of Lake Ontario. Am. J. Sci., 39: 651-656. Kirchgasser, W. T., 1973. Lower Upper Devonian stratigraphy from the Batavia-Warsaw meridian to the Genesee Valley: goniatite sequence and correlation. In: Guidebook N.Y. State Geol. Assoc., 45th Annu. Meet. Brockport, pp. C1-C21.

192 Kirchgasser, W. T. and House, M. R., 1981. Upper Devonian goniatite stratigraphy. In: W. A. Oliver and G. Klapper (Editors), Devonian Biostratigraphy of New York, Part 1. Text. Int. Union Geol. Sci. Subcomm. Devonian Stratigr., Washington, D.C., pp. 39-56. Klapper, G., 1981. Review of New York conodont biostratigraphy. In: W. A. Oliver and G. Klapper (Editors), Devonian Biostratigraphy of New York, Part 1. Int. Union Geol. Sci. Subcomm. Devonian Stratigr., Washington, D.C., pp. 57-66. Lafond, E. C., 1962. Internal waves. In: M. N. Hill (Editor), The sea, Vol. 1. Wiley-Interscience, New York, N.Y., pp. 731-755. Liebau, A., 1980. Pal~obathymetrie und Okofaktoren: Flachmeer Zonierungen. Neues Jahrb. Geol. Pal~ontol. Abh., 160: 173-216. Loomis, F. B., 1903. The dwarf fauna of the pyrite layer at the horizon of the Tully Limestone in western New York. Bull. N.Y. State Mus., 69: 890-920. Love, L. G., 1971. Early diagenetic polyffamboidal pyrite, primary and redeposited, from the Wenlockian Denbigh Grit Group, Conway, North Wales, U . K . J . Sediment. Petrol., 411038-1044. Lundegard, P. D., Samuels, N. D. and Pryor, W. A., 1980. Sedimentology, petrology and gas potential of the Braillier Formation: Upper Devonian turbidite facies of the central and southern Appalachians. U.S. Dep. Energ., Morgantown Energ. Tech. Cent., 5201-5, 220 pp. Mausser, H. F., 1982. Stratigraphy and sedimentology of the Cleveland Shale (Devonian) in Northeast Ohio. Thesis. Case Western Reserve Univ., 116 pp. (Unpublished). Maynard, J. B. and Lauffenburger, S. K., 1978. A marcasite layer in prodelta turbidites of the Borden Formation (Mississippian) in eastern Kentucky. Southeast. Geol., 20: 47-58. McCave, I. N., 1967. A stratigraphical and sedimentological analysis of a portion of the Hamilton Group (Middle Devonian) of New York State. Thesis. Brown Univ., Providence, R.I., ~381 pp. (unpublished). McIver, N. L., 1970. Appalachian turbidites. In: G. W. Fisher et al. (Editors); Studies:of Appalachian Geology: Central and Southern New York. New York Interscience, New York, N.Y., pp. 69-81. McIlreath, I. A. and James, N. P., 1980. Carbonate slopes. In: R. G. Walker (Editor), Facies Models. Geosci. Can. Rep. Ser., 1: 133-143. Park, W. C. and Weiss, A. J., 1972. Diagenetic sulfides in fossiliferous marcasite (upper Hamilton Group) from Devonian of New York. Abstr. Program Geol. Soc. Am., 4: 38. Reaves, C. R., 1984. The migration of iron and sulfur during the diagenesis of marine sediments. Thesis. Yale Univ., New Haven, Conn., 413 pp. (Unpublished). Rich, J. L., 1951. Three critical environments of deposition, and criteria for recognition of rocks deposited in each of them. Bull. Geol. Soc. Am., 62: 1-20. Rickard, L. V., 1975. Correlation of the Silurian and Devonian rocks of New York State (Map, Chart Ser.). N.Y. State Mus. Sci. Serv., 24, 16 pp. Rickard, L. V., 1981. The Devonian System of New York State. In: W. A. Oliver and G. Klapper (Editors), Devonian Biostratigraphy of New York, Part 1. Int. Union Geol. Sci. Subcomm. Devonian Stratigr., Washington, D.C., pp. 5-22. Rollins, F. O., Powell, C. A. and Dennison, J. M., 1984. Flexural modeling of Devonian Catskill Delta in Eastern United States and the formation of the Taghanic Unconformity. Abstr. Program Geol. Soc. Am., 16(3): 191. Seilacher, A., Andalib, F., Dietl, G. and Gocht, H., 1976. Preservational history of compressed Jurassic ammonites from southern Germany. Neues Jahrb. Geol. Pal/iontol. Abh., 152: 307-356. Seyfert, C. K. and Sirkin, L. A., 1979. Earth History and Plate Tectonics. Harper and Row, New York, N.Y., 2nd ed., 600 pp. Sholkovitz, E., 1973. Interstitial water chemistry of the Santa Barbara basin sediments. Geochim. Cosmochim. Acta, 37: 2043-2073. Simpson, P. R. and Bowles, J. F. W., 1977. Uranium mineralization of the Witwatersrand and Dominion reef systems. Philos. Trans. R. Soc. London, A 286: 527-548. Southard, J. B. and Cacchione, D. A., 1972. Experiments on bottom sediment movement by

193 breaking internal waves. In: D. I. P. Swift, D. B. Duane and O. H. Pilkey (Editors), Shelf Sediment Transport: Process and Pattern. Dowden, Hutchinson and Ross, Stroudsburg, Pa., pp. 83-97. Stanley, D. F. and Wear, C. M., 1978. The ~'mud-line": an erosion-deposition boundary on the upper continental slope. Mar. Geol., 28: M19-M29. Stanley, D. F., Addy, S. K. and Bahrens, E. W., 1983. The mudline: variability of its position relative to shelfbreak. In: D. J. Stanley and G. T. Moore (Editors), The Shelfbreak: Critical Interface on Continental Margins. Spec. Publ. Soc. Econ. Paleontol. Mineral., 33:279 298. Sutton, R. G., 1951. Stratigraphy and structure of the Batavia Quadrangle (New York). Proc. Rochester Acad. Sci., 9: 348-408. Sutton, R. G., Bowen, Z. P. and McAlester, A. L., 1970. Marine shelf environments of the Upper Devonian Sonyea Group of New York. Bull. Geol. Soc. Am., 81: 2975-2992. Tanabe, K., Inazumi, A., Tamahama, K. and Katsuta, T., 1984. Taphonomy of half and compressed ammonites from the lower Jurassic black shales of the Toyora area, west Japan. Palaeogeogr., Palaeoclimatol., Palaeoecol., 47: 329-346. Thayer, C. W., 1974. Marine paleoecology in the Upper Devonian of New York. Lethaia, 7: 121-155. Thomsen, E. and Vorren, T. O., 1984. Pyritization of tubes and burrows from Late Pleistocene continental shelf sediments off North Norway. Sedimentology, 31: 481-492. Uyeno, T. T., Telford, P. G. and Sanford, B. V., 1982. Devonian conodonts and stratigraphy of southwestern Ontario. Bull. Geol. Soc. Can., 332, 55 pp. Van der Voo, R., 1982. Pre-Mesozoic paleomagnetism and plate tectonics. Annu. Rev. Earth Planet. Sci., 10: 191-220. Van Straaten, L. M. J. U., 1967. Solution of aragonite in a core from the southeastern Adriatic Sea. Mar. Geol., 5: 241-248. Woodrow, D. L., 1983. Topography and sedimentary processes in an epicontinental sea. In: D. J. Stanley and G. T. Moore (Editors), The Shelfbreak: Critical Interface on Continental Margins. Spec. Publ. Soc. Econ. Paleontol. Mineral., 33: 159-166. Woodrow, D. L., 1985. Paleogeography, paleoclimate, and sedimentary processes of the Late Devonian Catskill Delta. In: D. L. Woodrow and W. D. Sevon (Editors), The Catskill Delta. Geol. Soc. Am. Spec. Pap., 201:51 63.