Palaeogeography, Palaeoclimalology, Palaeoecology, 74 (1989): 297-326 Elsevier Sc~encePublishers B.V..Amsterdam - Printed in The Netherlands

BATHYMETRY AND PALEOECOLOGY OF SILURIAN BENTHIC ASSEMBLAGES, LATE LLANDOVERIAN, NEW YORK STATE

BEA-YEH ECKERT' and CARLTON E. BRETT

Department of GeologicaI Setences, University of Rochester, Rochester, NY 14627 (U.S.A.)

meceived June 21, 1988; revised and accepted, May 1, 1989)

Abstract

Eckert, B.-Y. and Brett, C. E., 1989. Bathymetry and paleoecology of Silurian benthic assemblages, late Llandoverian, New York State. Palaeogeopr., Palaeoc~imatol.,Palaeoecol., 74: 297-326.

Late Llandoverian marine shales in New York State are highly fossiliferous and provide an opportunity to examine gradients of biofacies. The Williamson and Willowvale formations (Llandoverian C,) were deposited during a major transgressive episode in and adjacent to the axis of a narrow a-SW trending, central Appalachian foreland basin. The Williamson Shale is a black to dark green, graptolite-bearing unit of the upper Clinton Group of western New York State. It grades eastward into gray fossiliferous shales and coquinoid limestones of the Willowvale Formation. Orientation analysis of graptolites (Monograptue clintonensz's) indicates paleocuments dominantly from the northeast to east. The presence of nppled, cross-laminated sandstone beds (tempestites) within dark gray, graptolitic facies of the lower Williamson indicates that the dark shale facies was deposited in dysaerobic to marginally aerobic water slightly below storm wave base. Poorly fossiliferaus green shales, higher in the Williamson, appear to reflect sI~wdepos~tion in dysaerobic water and suggest a gradual deepening. Graded brachiopod coquinoid limestones, interpreted as storm layers, occur near the middle and top of the WiIliamson and In the Willowvale Shale, and indicate deposition of these sediments above maximum stom wave base. The faunas of the Williamson and Willowvale shales are subdivislble into five vertically and laterally adjacent associations as follows: (a) Ichnofossil (Chondrifes), @) Monograptid-"Chanetes" cornutus, (c) Eoplectodonta- Atrypa, (d) PaIaeocycIus-bryozm, and (e) Eoeoelia-"Chonetes" cornutus associations. This spectrum of fossil associations appears to parallel the increasing frequency of bottom disturbance by waves and currents &om deeper basirral areas to higher-energy outer and low energy inner shallow, shelf settings. The fossil associations of the WilIiamson (a-c) probably belong to the deeper benthic assemblages 4-5, but developed in environments withln to slightly below storm wave base, (about 3-80 m deep). The Willowvale fauna (c, d) representing benthc assemblages 34, inhabited somewhat shallower, better oxygenated environments of the photic zone. probably at depth of approximately 20750 m. The easternmost to slightly southeastward, equivalent of Williamson-Willowvale fauna (e) evidently represents shallow water benthic assembIage 2. These conclusions indicate the need for a critical reevaluation of the bathymetric reIations of benthic assemblages 3-5; a new interpretation of approximate depths for this range is presented herein.

Introduction for understanding the distributional ecology of paleoco~nmunities(Ziegler, 1965; Ziegler et al., Early Silurian fossil assemblages in the 1968). However, numerous problems remain Welsh BorderIand provided important models unsolved with regard to the paIeobathymetric relationships of the "classic" Silurian depth- EPresentaddress: 16 West Hampton &&, st. Catharines, related co-uniti@s. For example, communi- Ont. L2' 3% (Canada). ties assigned to the benthic assemblage 4-51

0031-0182/89/$03.50 0 1989 Elsevier Science Publishers B.V. range (e.g. Clorinda and Dieoelosia) are gener- The green clay shale Iithofacies of the ally accepted to represent offshore or rela- Williamson very closely resembles that of tively deep water settings. But does this mean certain Lower Silurian units (e-g. Sodus and the subpbotic zone, "shelf break", or below Maplewood shales) which display evidence maximum storm wave base? Does "deep water" of shallow water deposition and this may imply 50 m, 100 m, 200 m (see Boucot, 1975; have caused problems in interpretation of Cocks and McKerrow, 1984; Johnson, 1987; the Williamson and Willomale shales. By Wang et al., 1987)? analogy to these facies and on the basis of The well known Silurian strata of New York graptolite communities, the assignment of both State have received relatively little paleo- the Williamson and Willowvale to the Eocoelia environmental study, until recently. h this biofacies of benthic assemblage 2 has been paper we present the results of our detailed implied, though not explicitly stated (Berry pa1eoecological study of the late Llandoverian and Boucot, 1972, p. 61; Boucot, 1975; see interval in New York State, in order to especially Boucot's fig.13 in which the word interpret paleoenvironments and place this Williamson apparently should be replaced by sequence into the context of better-studied Willowvale). However, the Williamson sedi- assemblages from the Welsh Basin. This study mentary setting and fauna indicate deposition presents a revised view of the bathymetric in considerably deeper water. By examining relationships of these classic benthic assern- evidence for storm deposition, photic zone blages. indicators, and regional relationships, we pro- The WilIiamson and WiIlowvale formations vide more objective criteria for the depo- of New York State (Fig.1) comprise dark gray, sitional environments and relative depths black and greenish shales and thin carbonates involved in these deposits. and sandstones deposited in the northern end Abundant aligned specimens of the grapto- of the Appalachian foreland basin during a lite Momgraptus in the Williamson permit major late Llandoverian transgression (see paleocurrent analysis. Oriented samples from Gillette, 1947; Lin and Brett, 1988, for detailed five major outcrops (Fig.2) of the Williamson discussion of stratigraphy and facies relation- have been used to determine circuIatory dy- ships). namics in the upper Clinton sea. The distribu-

Fig.1. General stratigraphic relationships of late LIandoverian to earliest Wenlockian in New York State; W' and Hem. =Westmoreland Hematite {after Lin and Brett, 19%). Fig.2. Locality map ofthe WilIiamson and Willowvale shales. Localities include: 1 = Genesee Gorge West;2= Genesee Gorge East; 3= Tryon Park (Palmer's Glen); 4= Salmon Creek West; 5= Second Greek: 6= Little Wolcott Creek; 7= fioenlx; 8= Brewerton: 9= Verona Station; I0= Lairdsville; I1 = Dawes Quarry Creek; I,?= Willowvale Glen; I3= Chadwicks; 14= Ohisa Creek; 15= Van Hornesville. Oriented Manogmptrrs samples were collected from localities I to 5. tion of Iithofacies, and taphonomic features in were deposited near the northeast end of the the Williamson and Willowvde shales, to- central Appalachian foreland basin (Johnson, gether with paleecurrent data, permit recon- 1987). Marine faunas of pelagic graptolites, struction of the depositional environment and brachiopods, trilobites, echinoderms and others paleogeography of the late Llandoverian were abundant near the margins of the basin. strata. Additionally, the distribution ef bracho- Most paleogeographic reconstructions of pods and other fossils has been recorded for all North America during the Silurian (Ziegler et measured sections (Lin, 1987); this has permit- al., 1977; Bambach et al., 1980; Johnson, 1987; ted recognition of several distinctive associa- Van der Voo, 1988) indicate that New York lay tions which can be related to other facies in a subtropicaI climate belt about 25-30" S of criteria and compared with the classic on- the paleoequator, which would have placed the shore-offshore gradient established for the epeiric seas in the pathway of tropical storms, Silurian in the Welsh Basin (Ziegler, 1965; equatorial currents, and trade winds. Con- Ziegler et al., 1968; McKerrow, 1978). versely, the reconstructions of Boucot and Gray (1983) and Boucot (1985) would place New General geological setting York in mid-southern latitudes, or in a zone of winter storms (Marsaglia and Klein, 1983). h The Lower Silurian Clinton Group of New any case, the WilIiamsan-Willewvale shales York State was deposited in a tectonic setting provide evidence for storm-influenced shelf transitional between orogeny and extreme sedimentation, as has been noted previously quiescence (Hunter, 1970). Terrigenous sedi- for the sIightly younger Rochester Shale ments derived from the Taconic orogenic belt (Brett, 1983). The Williamson-Willowvale shales repre- sent the initial deposition of a widespread latest Llandoverian (C,) marine transgression (Johnson et d., 1985; Johnson, 1987) and overlie a widespread, regionally angular ero- sion surface (Lin and Brett, 1988). According to Johnson et al. (1985) this sea level peak was the last of three Early Silurian large scale trans- gressions which occurred with a periodicity about 2.5 million years.

Williamson-Willowvale paleoenvironment and facies

Paleogeography and paleocurrent analysis

General facies geometry suggests that dur- Rg.3. Depositional setting of the late Llandovenan ing deposition of the Williamson Shale the axis Williamson and Willowvale shah. Eastward. gradation of maximum subsidence of the central Appala- from black and sparsely fossiliferous green shales at the chian Basin lay in western New York and type section eastward into thicker, grayish shales with western Pennsylvania. This basin extended coquinites indicates that the basin axis was in west-central New York, with water shallowing eastward. This pattern southward into western PemsyIvania and is also indicated by paleocurrent and fossil community Ohio, where the Williamson grades into the studies. upper Rose Hill and EstilI shales (Hunter, 1970; Brett and Goodman,1988) (Ag.3). Closure or at least shaIIowing of the north end of the basin indicator (Nagel, 1967; Jones and Dennison, (Johnson, 1987), north of the present outcrop 1970; Potter and Pettijohn, 1977). GraptoIites belt is suggested by E-W trending facies belts are light and elongate; their rhabdosomes with evidence for deeper water deposits in the probably responded to the most gentle of New York subsurface in many Silurian units currents, aligning with the long axes of stipes (Rickard, 1969; Belak, 1980; Brett, 1983), as parallel to the flow direction; as a result, they well as evidence for minor siliciclastic sedi- provide sensitive indicators of paleocurrent ment input from the north (Muskatt, 1972; systems (Ruedernann, 1897; SchIeiger, 1968). CrowIey, 1973), and the absence of Silurian FIow directions during deposition of the deposits at Montreal (Boucot et al., 1987). The WilIiamson Shale were inferred from aligned eastern shoreline during latest Llandoverian graptolites (Monograptus, Retiolites) and sole probably Iay somewhat to the east of Herkimer marks. Computer-aided analysis of over 1200 County, N.Y. area where the Willowvale Shde oriented Monograpttls rhabdosomes from black is now beveIIed due to a Late Silurian erosion shales at five Iocalities indicates rather consis- interval (Rickard and Zenger, 1964). Eastern- tent current alignment. Approximately equal most outcrops of the Willowvale .yield a proportions of Monograptus rhabdosomes are distinctive nearshore (BA-2) fossil assemblage oriented with the thecae pointing to the left or dominated by the Eocoelia brachiopods. to the right. GraptoIites on bedding planes Analysis of aligned graptolites suggests a show significant to highly significant preferred NE-SW orientation of the eastern paleoshore- long axis orientations (Rvalues ranging &om Iine during the deposition of the WiIliamson- 0.63 to 0.95); the null hypothesis (random Willowvale shales (Lin, 1987). Fossil orienta- orientation) was rejected with a Chi-squared tion has been widely used as a paleocurrent test in 11 of 12 sampled horizons. Orientation is typically in the range of N 20"E to due north. (Loc. 3) and Second Creek (Loc. 5) reveal a As a rule, long rhabdosomes (4 cm or longer) dominant northeast to southwest or north to show a higher degree of preferred orientation south flow direction. than do shorter rhabdosomes,which apparently Concretions in the Williamson at Genesee were subjected to reworking and fragmenta- Gorge East (Loc. 2A) also display aligned long tion (Fig. 4). Some variation in orientation is axes oriented N 50°E, a direction consistent evident between localities and from horizon to with the aligned Monograptus (see Fig.83). horizon at each locality. Furthermore, a few These concretions ocmin a discrete horizon bedding plants preserve graptolites oriented immediately above a thin, laminated siltstone mainly at approximately right angles to the and were probably formed by selective cemen- dominant trend displayed at most horizons. tation along zones of high permeability. Colton This seems to indicate a current flow approxi- (1967) and Craig and Walton (1962) previously mately perpendicular to the dominant flow. have noted that elongate concretions commonly Aligned graptolite rhabdosomes display ori- paralleI transport directions within turbidites. entations consistent with those of sedimentary The paleocurrents were probably domi- structures (cross laminae, ripples, soIe marks) nantly gradient currents, approximately per- and were produced by the same unidirectional pendicular to the marg-ins of the basin but a paleocurrents. Sole marks preserved at the minority were parallel to the basin contours. bases of graptolitic horizons at Tryon Park One interpretation is that most currents in western New York Aowed approximately N to S, indicating a gentle paleoslope toward the S. A regional trend of current orientations dis- plays minor change from consistentIy SSW at Mton &oc. 5), to more due S or even SE in the Rochester area. This suggests that, from E to W, the contours of the basin margin might have curved gently from NW to more due W or even slightly SW (Figs.5 and 6). The fact that both SE and SW orientations occur in equal abundance at Salmon Creek is consistent with other indications that this section Iay near the basin axis. No graptoIite orientation data were obtained horn rocks east of Alton. However, previous paleoarrent studies based on ripples and cross beds fiom the middle Clinton Group near Utica, indicate E to W or NW-flowing currents (mean composite cross-bedding dip azimuth for Clinton sandstones: 284" 64"; Muskatt, 1972). This may indicate a general SE to NW orientation of the eastern side of the basin. Our interpretation of the variation of graptolite orientation from AIton to William- son to Rochester is that the WiIliamson basin formed a narrow trough trending roughly Fig.4. Size frequen~yanalysis of Momgmptus graptoliteg NE-SW (Figs.3 and 6); the northwest rim of of the Williamson Shale for several localities. The left- this basin trended ENE to WSW, appraxi- skewed pattern of the Momgraptws rhabdosomes indicates that the rhabdosomes probably underwent some transport- mately parallel to the present Silurian outcrop ation, reworking and hgmentation. belt kern Rochester westward. This pattern is is extremely thin, or is absent in western New York (Lin and Brett, 1988), suggesting that the northwestern side of the basin was sediment- starved. Maximum development of the dark gray to black graptolitic facies occurs in a narrow area between Williamson and Tryon Park, and this region is inferred to represent TP-U TP-M TP-L the basin center. In more easterly localities, olive to greenish gray clay shales and coquinoid limestone with abundant fossils dominate the upper part of Williamson. These greenish gray shales facies SAM-U ' SAM- L SC-U pass eastward into gray slightly silty shales of N tQ'E the WillowvaIe F~rmation. Phosphatic and quartzose conglomerate beds (Second Creek Bed) 2-5 cm thick in the basal 30 cm of the Williamson at most of the locali- SC- L1 ties and represent a transgressive lag deposit 16'E associated with the basal unconformity (see Lin and Brett, 1988); they are immediately overlain by dark gray to black shales with abundant Monograptm. The Williamson is coarsest at the base and gradually fines up- wards into fine, greenish gray clay shales. Fig.5. Azimuthal rose diagram indicak general trends in These beds, in turn, pass upward into a middle graptolite orientation; note NE-SW orientztions at sev- minor interval with minor black shales and erd localities. GG-S (Genesee Gorge West, unit 16); GGE (Genesee Gorge East, Location A; unit 7); GGE2 [Genesee sandy coquinite. The Williamson terminates at Gorge East, Location 3;unit I); TP-U,TP-M, TP-LQ!ryon a phosphatic and quartz pebble-bearing, dolo- Park; units 18,16,9),SAM-U, SAM-L (Salmon Creek West, mitic limestone bed which marks the base of unit 1; upper and lower horizons), SC-U, SC-M, SC-L, SC- the Irondequoit Formation. Greenish gray clay L1 (Second Creek, units 16,3,2) (see ;ti1987, for detailed measurement of sections and description of methods). shales with a few poorly preserved brachiopods and triIobites and typically lacking Mono- graptus form the bulk of the upper part of the consistent with basin geometries, previously Williamson Formation, with recurrent black noted, that suggest closure of the basin and beds in the upper third of the unit at some shdlowing to the north. localities. On the basis of provenance studies, Muskatt Lithofmks and cyclicity (1972) concluded that sediments comprising the upper Clinton Group were derived dorninant1y Eastward thickening of the Williamson £rom source areas to the east or southeast, Shale and gradation into slightly coarser composed of low grade metamorphic rocks grained, fully aerobic facies of the Willowvale with abundant quartz veins as should be Shale indicate a cIastic provenance and gen- typical of Taconic terranes. However, he also eral shallowing to the east. The aggregate notes the presence of some grains derived from thickness of the quartzose sandstone,siltstone sedimentary rocks, gneiss, pegmatite and pos- and coquinoid limestones and thickness of sible granite and basalt (Muskatt, 1972, individual beds of these lithologies increase p. A27). Abundance of sand- and silt-sized toward the east side of basin. The WilIiamson siliciclastic grains at the base of the WiIIiam- Fig.6. Map showing average orientations of graptoliks at various locations and inferred basin contours (relative). In general, graptolites are oriented mainly N-S in Rochester area, but show a major distinctly NF-SW orientation IU more easterly samples. These orientations were probably pruduced by storm-generated currents flowing perpendicular to the basin contours; hypothetical basin contour lines are shown in dotted Lines; orientation is approxmately perpendicular to mean vectors for paptolite azimuths. Th~esame currents probably transported sand into the basin. son indicates that the basin center was receiv- Lin and Brett, 1988). Hence, the Williamson- ing sediment from reworking of an eroded WillowvaIe interval, as a whole, constitutes emergent terrain, probably near the eastern two subsymmetrical transpessive-regressive and perhaps northern borders of the basin cycles. While the water was still relatively shallow, some coarser sediments were transported into Biofacies and paleoecoZogy of the the basin probably by intermittent storm- Williamson-Willowvale shales generakd gradient currents (see below). With gradual deepening, and transgression of shore- Fossil associations, as defined herein, are Iines the western side of the basin was cut off recurrent groupings of fossil taxa; they may from receipt of coarse siliciclastics. or may not represent true communities; for

The frequency pattern of storm-generated example the Monograptus- "Chonetes" cornu- beds (tempestites) during deposition of the tus association contains a mixture of benthic Williamson dso suggests a central minor elements and pelagic graptolites. Conse- regression. Locally phosphatic brachiopod co- quently we avoid the term "communities" and quinoid limestones and sandstones occur use the broader and less genetic term associa- within the middle of the WiIlowvale Forma- tions. tion. Sparsely fossiIiferous green shales, be- tween shell rich intervals, appear to reflect Chondrites association.-greenish gray shale slaw deposition in dysaerobic water and sug- facies gest a graduaI deepening. A shallowing episode at the end of the Williamson-WiIIowvde (I) Foss i bs intervaI is recorded by the recurrent quartz Beds displaying abundant small-diameter granule-phosphatic bed (Salmon Creek Bed; Chondrites, alternate with laminated layers in -2 crn the basal Williamson Shale at Genesee Gorge deformed outlines of the burrows also indicate West (Loc. 1) and Tryon Park (Loc. 3). A at least two-fold compaction of muds (Fig.7C). polished core of the Williamson Shale also Small, pyritic, threadlike burrows associated revealed abundant, small diameter, flattened with greenish Chondrites-bearing shales ap- burrows throughout much of the upper green pear to represent mucous-lined burrows that shales (Fig.7C). Body fossils are quite uncom- underwent selective early diagenesis. mon in most horizons, although some bedding The color of these shales indicates a low planes contain poorly preserved moIds of content of organic carbon and of iron sulfides brachiopod valves which apparently dissolved relative to the dark gray shales (Potter et al., prior to compaction. 1980). The greenish shale facies of the William- son is usually devoid of body fossils, although (2) Sedimenfology and taphnomy this may partly reflect early dissolution (see This facies is composed of platy greenish below). Moreover an abundance of minute gray (5GY 4/1), clay shales generally lacking Chondsites (~i~s.7~and 8C) in these shale interbeds. Pyrite occurs only rarely as thin layers indicates low oxygen (dysaerobic) condi- linings of thread-like burrows. Burrows are tions near the substrate (Bromley and Ekdale, generally not apparent but very fine Chon- 1984; Savrda and Bottjer, 1987a). Horizons with dsites occur as vague, buff colored areas on Chondrites in otherwise unbmowed shales some weathered portions of the shales. indicate temporary sIight oxygenation of the seafloor. As a whole, the trace fossils within (3) Paleoautecology the Williamson represent offshore environ- Chondrites may represent endiclmial-mining ments probably in depth ranges well below traces (fodinichnia), that show repetitive, sys- wave base and rarely affected by storms. The tematic and branched patterns. Alternately, absence of Chondrites in laminated shales Bottjer et al. [in press) argue that some containing well-preserved graptoIites may Chondrites may represent tubes produced to reflect anoxic conditions or very rapid suspen- pump anaerobic bacteria fiom pore waters. sion ad redeposition of sediments which obliterated traces and laid down layers that (4) Interpretation were too thick to be later penetrated by Chondrites and pyrite stringers, ahernate burrows. Pervasive burrowing of the upper with laminated beds of dark gray graptolite- WiIliamson beds indicates reIatively sIow de- bearing shaIes in the basal beds of the William- position which allowed fairly thorough bio- son. Burrows in light green matrix are com- turbation. monly darker than surrounding shale; those in Teichichnus and indeterminate pyritic bur- dark matrix are typically Iighter. This contrast rows ocm in thin (2 cm thick) beds of pale appears to reflect piping or backfilling of Iater greenish sandstone within greenish shales sediment into older muds. However, strongly (Fig.7A7B). SrnaII pyrite nodules and pyritic

Fig.7. Stratification and trace fossils of the Williamson Shale. A Teichiehnw trace fossil &om unit 11 at Genesee Gorge East (Loc.2A). B. Polish& slab contains TeichiehrsusassociaM with pale greenish sandstone with green shale partings hmunit 11 at Genesee Gorge East (Loc. 2A). C. Polished $rill core displays distinctly compacted trace fossils; upper part of the Williamson Shale from Rochester area. I3. FoLished slab of concretion displays internal lamination; surrounding shales exhibit approximately tw*fold differentid compaction rehtive to concretion; unit 5 at Genesee Gorge East (hc. 24. E. Sharp base of the coquinoid Iimestones with gutter casts or scour marks; unit 38; Second Creek (Loc. 5). F. Polished core displays intimately interbdded greenish clay shales and black, laminaM shales. (arrow indicates sharp contact; note convex down shell of brachiopod); Rochester area. G. Polished slab of finegrained sandstones and laminated siltstones, unit 33; Second Creek Gc.5). H. Basal conglomerate bed at Second Creek @c. 5); arrow shows the contact of Williamson Shale and Wolcott Furnace Shales. A 2 cm

2 crn I crn burrows are associated with the Teichichnus ant than the green clay shales and may occurrences. The distribution of these facies weather from the bank as ledges. Bases of the was probably controlled by greater oxygena- dark bands are commonly sharp and display tion andlor the deposition of coarser sand evidence of erosion of the underlying units substrates. (Fig.7H). Individual black bands are correlat- able for distances of tens of kilometers (see Lin Monograptid-"Cbnetes" cornutus and Brett, 1988). The dark shales contain association: dark gray to black shale facies minor pyrite aggregates in the' form of small (usually 1-2 mrn) spheroids, as drusy coatings (I) Fossils an bedding planes, or rarely as infillings of The Monograptid- "Chonetes" cornutus as- fossil nautiloids and graptolite rhabdosomes. sociation contains abundant Monograptus Dark shales are commonly interbedded with clintonensis and a few Retiolifes graptolites, thin (0.5-1.0 cm)to very thinly-laminated, fine- brachiopods, including small specimens of grained quartz arenites (F4g.7G). Thin sections Eoplectodonta transversalis, Atrypa reticularis, of these sandstones and siltstones show thin and Leptaena "'rhomboidalis", "Chonetes"cornu- quartz sandlsilt laminae (<2 mm thick), alter- tw, Skenidioides pyramidale, Whitfieldella ? sp. nating with layers of muds containing bie- (one bed only), the ostracodes Beyrichia, Mati- clasts ( < 1mm in diameter) possessing abundant gobolbina typus and Dibolbina (Table I). Or- pyrite coatings (Fig.9A). Laminated beds from thoconic nautiloids and tentaculitids are found Tryon Park display reworked, aligned pyr- immediately below the graptolite-bearing tized burrow fillings and brachiopod valves on shales in the Genesee Gorge but maybe lower surfaces (FigAA), immediately overlying reworked from the underlying Lower Sodus graptolitic shales. Abundant disseminated late Shale in which they are abundant (Fig.8D; diagenetic pyrite is present. TabIelJ. Very abundant individuals of the GraptoIites vary in completeness &om brachiopod "'Chnetes" cornutus, are found in nearly entire pyritized rhabdosomes to carbon- some of the graptolite-bearing layers. GiIIette ized fragments. They are typically found in (1941) also reports Closinda fornicata and dense aggregations displaying preferred orien- Chaetetes Iycoperdon at Rochester, but these tation of individuaIs with dominantly uni- reports could not be confirmed. modal distributions (see above). Bedding planes commonly bear abundant fossil debris (2) Sedimentology and taphonomy including small specimens ( < 1 cm) of Eoplecto- The Monosaptid-"Chonetes" cornutus asso- donta, Atrypa and chonetid brachiopods that ciation occurs in dark gray to nearIy Mack may be partly or wholly replaced by pyrite. (N3-N1) or rarely dark greenish gray (5GY Thin, pyritic, threadlike burrows (less than 5/1), fissiIe to platy, laminated shaIes. This 2mm in thickness) occur in the basd 2 cm facies ranges from smooth day shale to thick, dark, graptolite-bearing shales; larger slightly silty shale. Dark bmds within the pyritic burrows in the dark gray shales are Williamson tend to be somewhat mare resist- restricted to higher horizons.

Ag.8. Sediments and fossils in the Williamson Shale. A. Dark gray shale with Eoplmtodonta brachiopods (a), reworked pyrite burrows (b), and tentaculitids; lower Williamson Shale, Tryon Park (Loc. 3). B. Thin layer displays convex upward Whitfieldelh (?) brachiopod valves (a),with lunate scour marks around their margins (b),unit 6; Genesee Gorge East (LOG. 24. It indicates the probable southwest flowing currents; penny gives scale. C. Slab shows small Chondriks burrows associated with Monogrupt~graptolites; Iower Wi3liamxon Shale, Tryon Park Gc.3). D. Slab shows oriented tentaculitids (a), ostracodes (b), and Eoplect&nla brachiopods (c); tower Williamson Shale, Genesee Gorge West @c. I). E. Monograptid-"Chonetes" association. Slab displays oriented Momgrtrptus (a)Retiolites (b); Lower WilIiamson Shale, Second Creek (LC.5). TABLE I

Fossil distribution in the Late Llandover~anWilliamson and Willowvale shales of New York and upper Rose Bill Formation of Pennsylvania. Symbols: M-"C": Monograptid-"Chnetes", E-A: Eoplectodontu-Atrypa, P-b: Pulueocyclw-bryozoan, E-"C*': Eocoelia-"Chowles". g: report by Gillette (1947), c: common, cc: abundant, s: report by Swartz (1934), x: present, z: report by Rickard and Zenger (1964). * probably reworked from Lower Scdus Shale into the basal beds of Williamson. RC: drill Core, Rochester; GG: Genesee Gorge, Rochester; SM: Salmon Creek west, Williamson; SC: Second Creek, Sodus; LW. Llttle Wolcott Creek, Wolcott; PH: Phoenix, Schroeppel; BB: Brewerton, Cicero; VR. Verona Station {Thruway cut), Verona; DQ: Dawes Quarry Creek, Kirkland; WV1: Willowvale Glen (upper), New Hartford: WV2: Willowvale Glen (lower), New Hartford; OH: Ohisa Creek, Stark; VH: Van Hornesville, Stark; PA: Rose Hill Formation (Mmtigobolbina typus zone), Pennsylvania.

Fossils Benthic assemblages

- - .- - -- - . Associations M-"C*' &A P-b &"c

Location RC GGI SM sc LW PH BR VR/ DQ WV~WVZ/ OH w PA Algae? (FLeceptaculitida} Ischadites planoconuexurn Porifera (Sclerospongidae) Chaetetes lymperdon Cnidaria Pugosaj Enterolasrrur cnliculum Palaeocycl~lsrotuloides flabulata) Famites Bryozoa Rcanthclemu asperum Cemmopora imbricata Eridodrypa sdriata Fenesdella elegans Mesotgvpa laurnmiformis Phy lloporina asperata-striata Rhinoporn wrrwosa Semicoxiniutn knuiceps Brachiopoda Atrypa reticularis xxgccc cxg g X S Atrypino dispurilis ggg = g "Chonebes" cornutus xcxgggg? Kg CCS Clorinda fornicata g? Coolinia subplana g? gx gg g X x Costistrieklandia Eihta X Cyrtia metu g g ggxg Dakjim hybd g X X X Dieoelosia biloba Kg? g g x g g Dolerodis flabella g X Emelia sulcata g? g? 1 E g g CC CC CC Eoplectodontcr transuersaIis xxgccccccxg g S Eospirifer radiatus gxgxgc gg g Hedeina eudora g s Leptaerul rhmboidalis ggxxggcgg g XXS LinguZa Iamellata g? g L. perovata 2 Z Nf6~leospirapisifomis g? g? "0rtFSi.s" &nu idem g g lz g PhoEidops squammifornais gxxx x g z Z Phtrypa nodostriada g # X g Protomegastrophia pmfunda g g TABLE I Icontinrsed 1

Fossils Benthic assemblages

Associations M- "C" E- A P-b E- "C"

RessereIla elegantula Rhynchonelb b&m Skenidioides pyramidalis Stegerhynchtss winus S. neglecturn "Strophodonta" corrugala S6mphoprion pa6enta Whttfildelb intermedia W. ?sp. cf. W.obbtu XXX W. oblata Bivalvia Cornellites emaceraturn

Crsneamya alueata Leptodesma rhmboidea Pyrenomoeus cuneatus Gastropoda Cyclonema uaricosurn Hormotoma subulata (sulcatum) Strophostylus cancelbtw Cephdopoda (Naukloidea) Dawsonocerm annubturn xgg Qrthn~~rasbassl~ g xg? Trilobites Dalmanites limulurus LimaLymene cltntoni L. niagarensis Ostracodes BeyrEchia aff. lakernontensis WiboEbina n. sp. Mmtigobolbim punctata M. triiobaia M. tpus Plethobolbirsa typkalis Zygosella cf. uallata Graptolithina (Graptoloidea) Monograptus ~Iintonensis CC CCC Retiolites venosus X XXX (Dendraidea) Dictyonem gracilis Reticalograptus retiform Incertae sedii Corrulena niagarensis Tentmulites sp. Crinoidea Periecbocrinitid sp. (Large root system) XXX x Protaxocrinus n. sp. X X Asteroidea Paleaster ? sp. x (Fig.7D). However, the Iaminae are more widely spaced within the concretions and converge toward the margins indicating approximately two-fold differential cornpac- tion of shales relative to concretions.

(3) Paleoautecology Life habits of Monograptus axe essentially unknown, but graptoloids are believed to have been epiplanktonic, or weakly nektonic. Mano- graptm rhabdosornes, though occasionally 2 mrn quite long (more than 20 cm), are invariably incomplete. Floats or holdfast attachments were not observed in this study. Most of the associated brachiopods e.g. "Chonetes" ccrrnu- tw and Eopkctodonta were redining epifaunal suspension feeders (see below).

(4) Interpretation The dark gray to black beds of the William- son appear to have been deposited mainly in restricted, oxygen deficient environments, with minor fluctuations into better oxygenated or anoxic conditions. The laminated, non- bioturbated nature of the lower dark beds indicates anoxic conditions within the sedi- ment to near the sediment-water interface. 2 mrn Certain beds also yield a low diversity benthic assemblage consisting mostly of small, low- Fig.9. Thin sections of mquinoid limestone and quartzow level suspension feeding brachiopods. This sandstones of the Williamson Shale. A. Quarhose sand- fauna indicates that minimally aembic condi- Second @c. B. Coquinite stone, unit 33; Creek 5). bed tions in bottom water. shown in Fig.7C displays a winnowed accumulation of prevailed the The chaotically oriented brachiopod shells with calcite cement general absence of infauna suggests exaerobic between the shells, Second Creek (hc.5). conditions (Savrda and Bottjes, 1987b cf. also the "bituminous'Yacies of Morris, 1979, 1980). A pelagic fauna of graptolites and perhaps As noted, light olive gray (5Y 5/21, ellipsoi- nautiloids lived much higher in the water dal, carbonate concretions with aligned long column. axes up to 10 cm long and up to 2cm in Dark gray to black shales of the basal thickness are found in the Iower part of the Williamson display beds containing Chondrites Williamson Formation of Genesee Gorge East that alternate with laminated sandy sediments (Loc. 24. They occur within laminated, dark (Fig.7C, F). The Chondrites layers represent gray, silty shales with a layer of Whitfieldella temporary oxygenation of the sea floor (0.1- (?$ brachiopods and very abundant specimens 1.0 ml oxygenll, dysaerobic environment; of Monagraptus at the top, and small (0.5 cm) Savrda and Bottjer, 1987a,b) and commonly pyrite nodules at the base. Concretions display supercede graptolitic shales deposited under alternating dark gray to black laminae, con- conditions of rapid sedimentation. Such storm tinuous with those in the surrounding shales layers may have been selectively colonized by the trace-makers (cf. VossIer and Pemberton, with intact soft tissue which provided a 1988). reductant for sulfate-reducing bacteria. At Ieast some graptolitic horizons may Geochemical studies demonstrate that much represent storm-generated sandlmud layers. pyrite forms in the uppes few meters of Association of graptolites with coarser sedi- sediment in direct contact with sea water ments could have resulted from sudden impor- (Fisher and Hudson, 2985; Raiswell and Ber- tation and burial of rhabdosomes. ner, 1985). Graptolites infilled with pyrite are The clustered distribution of well preserved inflated and robust in contrast to other coal- graptolite rhabdosomes (FigSE) suggests mass ified specimens which are highly compressed; mortality, which could have resulted from simiIarly the pyritized nautiloids show lesser overturn or upwelling of anoxic water during degrees of compaction than nonpyritic speci- storms. Graptolites then sank to the bottom, mens and display brittle fracturing of pyrite were subject to varying degrees of transport- linings. Moreover, there is evidence for ation, reworking, and concentration and were reworking of pyiitic burrow tubes on the subsequently buried. seafloor. Thus pyritization within this facies Alignment of graptolite rhabdosomes sug- probably developed very early, prior to com- gests that mass mortality and rapid burial were paction (burial of about 5 m; Allison, 1988), in accompanied by moderate current action on response to local, organic-rich microenviron- the seafloor. Fragmentation of graptolites in ments in otherwise moxie but organic-poor, some beds may indicate brief periods of rework- nonsulfidic sediments (see Brett and Baird, ing of skeletons on the seafloor. Erosion of the 1986; Allison, 1988 for discussion). seafloor by the same currents is indicated by Berry and Boucot (1972) considered the scour marks and the presence of reworked Moltograptus priodon-Retiolites genitzianus pyrite clash derived from underlying muds. association to be a shallower water graptolite This evidence indicates that the quartzose biofacies-probably belonging to BA-2 or BA-3 sandstones and siltstones and interlknated (i.e. inner shelf muds associated with Eocoelia graptolitic shales were deposited by episodic biofacies); they cited the Williamson and strong currents in otherwise Iow energy an- Willowvale shales as a typical example of this aerobic to dysaerobic environments. Con- type of occurrence. However, all evidence sistent long axis aIignments of graptolites presented in this paper (including brachiopod pardIe1 to basal tool marks indicate that the biofacies) indicates that the Williamson was entire bed of interlaminated hesand and mud deposited in deeper, offshore settings, probably was laid down during a single current emanat- near the center of a density-stratified basin; the ing from the N to NE. Evidence for differential associated brachiopods-(EopJectodonta, "'Cho- compaction around concretions suggests that netes", Dicoelosia, Skenidioides) would favor a dark gray muds and silts were originally water- BA-4 to BA-5 assignment, comparable to many rich. graptoIitic shah of the Welsh Basin. Sedirnen- Occurrence of abundant graptolites, in part, tological evidence also indicates that these certainly reflects a preservational bias. The sediments accumulated in environments episo- organic-wdIed rhabdosomes of graptolites may dically affected by stom-generated gradient have been disintegrated by scavengjng and currents, but not directly by storm waves. possibly bacterial action; they are rarely found We do not mean to imply that Monograptu in fully oxic sediments. The preservation of and Retiolites represent deep water graptolites. graptolites was considerably enhanced by The shallow water: occurrence of these grapto- burial in anoxic, organic-rich sediment. Early lites has been further conhed by Mu et al. pyrite diagenesis also aided in preservation of (1986); and, as noted below, they do occur in some Williamson graptolites. Such pyritization relatively nearshore, shalIow water facies of may have resulted from burial of rhabdosomes the Willowvale as well as the offshore WiIliam- 2 cm lL!L

I crn

t crn son beds. This is the expected distribution the trilobite Liocaiymne clintoni (Fig-lQD), pattern of a shaliow-floating pelagic organism; paorly preserved bivalves and nautiloids, the the rhabdosomes probably settled after death ostracodes Beyrichia, Dibolbina and Mastigo- uniformly to shallow as we11 as deeper water bolbina, and rare specimens of the solitary seafloors. The abundance of the monograptid- rugose coral Enterolasma; a single articulated "Chonetes" cornutus association in offshore, starfish was found in the uppermost part of dark gray Williamson facies probably reflects Williamson (Table I). Thn dark layers with primarily favorable circumstances. Unex- abundant, mostly fragmented Monogsap tus are plained at present is the absence of deeper interbedded with the Eoplectodonta-Atrypa water graptolites from the Williamson, and all association. Slabs of olive gray shale identified other SiIurian offshore facies in the central as "Willomale Shale from Verona Station, Appalachian Basin. Possibly the larvae of N.Y." in the New York State Museum, display these graptolites floated deeply and were bedding planes covered with the receptaculitid excluded from entering the Appalachian Basin Ischadites planocbnvexa (see Nitecki, 1972). by surrounding shallow water platforms; in Associated brachiopods on these slabs suggest contrast shallow floating larvae of monograp- the Eoplectodonta-Atrypa association. We tids and various brachiopods were not ex- have recently discovered specimens of Ischad- cluded (see also Boucot, 1975). ites horn a similar litho- and biofacies in blocks of probable upper WiIliaroson ShaIe and in the Eoplectodontu-Atrypa association: gray shale lower beds of the Irondequoit Formation, facies which overlies the Williamson, and contains a Plectodonta and Clorinda dominated fauna. (1) Fossils This is primarily a brachiopod-dominated (2) Sedimentology and taphonomy association, with moderate diversity assem- The Eoplectodonta-Atrypa association oc- blages (species richness 20-25 species in rich- curs mainly in the olive gray (5Y 411) to blueish est beds; Table I). The brachiopods Pholidops gray (5B 5/11 shales, and in interbedded shell- squammiformis, Eoplectodonta Eransuersalis, supported coquinoid limestones (Fig.TIA-C). Afrypa reticula^, Leptaena rhornboidalis, Brachiopods from dark greenish gray shales Coolinia sp. and "Orthis" tenuidens are locally are commonly moldic or partially dissolved. very abundant. Less common species include They are predominantly in convexdown orien- Atrypina disparilis, Cyrtia meta, Eospirifer tations (56-85%) (see Appendix I) and the rdiatus, "Chonetes" curnutm, "Strophodontan valves contain mud consistent with surround- corrugata, Resserella ekgantula and Rhyncho- ing matrix. The high percentage of convex nellu bidens (see Gilette, 1947). GilIette (1947) down orientation of brachiopod valves sug- reports abundant Dicoeosla biloba and Skeni- gests burial in life position, or by settlement dioides pyramidalis, although these minute after stirring from the bottom by waves. orthid brachiopods appear to be coniined to Coquinites are scattered, commonly lenticular, thin layers in the uppermost beds of the bluish gray (5B 5/11 beds, 2-4 crn in thickness, WilIiamson Shale associated with Eoplecto- mainly composed of the brachiopods Atrypa donta and Atqa. Associated fossiIs include and Eopkctodonta. Elongate lenses of bra-

Fig.10. Fossil associations of the Williamson-Willowvale shales. A-D. Eoplectdonta-Atypa association; unit 39, Second Creek, Alton, N.Y. @c. 5). A. Note abundant Atrype sp. (a) and Eopbctodonta transversalis (b) brachiopods. &. Shows reclined Atrypa with extensive 631s (a), Eopkctdonta (b), and small orthid brachiopods (c}. C. Leptaena rhomboidalis brachiopod. D. Articulated kiIobite Liocalymene clintoni. E. Padaeocyclus-bryozoan association; Paiaeocyclus comb (a), and fenestellid and ramose bryozoms (b); Willowvale Shale, 'She Glen, Chadwicks, N.Y. aoc. 12). F. Specimen of the large brachiow Costistriekbndia Iiratu; Willowvale Shale, Verona Station, N.Y. (lac. 91, white bm=4 cm. reveal abundant pyrite coatings on disarticu- lated brachiopods within the coquinoid lime- stones (Fig.93). Pyritic burrows are also found in greenish gray shales associated with these coquinoid limestones.

2 crn (3) Paleoautecology This association is dominated by low-level epi- faunal to semi-infaunal brachiopods (Fig.lOA- C). Atrypa reticularis, which is commonly the dominant species, probably reclined on its pedicle valve; the presence of broad, marginal frills on specimens in the upper Williamson suggests an adaptation for increased bearing area, in response to soft substrates. Eopleeto- donta, Leptaena and Coolinia were also adapted to a very low epifaunal to semi- infaunal mode of Iife and probably rested on their convex pehcle valves. These brachiopods were well adapted to soft plastic muds; evi- dently they were able to cope with relatively low oxygen levels, as they also occur in the dark gray nonbioturbated shales of the Wil- liamson. High level suspension feeders, such as bryo- mans and crinoids, generally are very rare in this association, perhaps reflecting paucity of suspended food or low oxygen leveIs andlor of turbulence for passive Fig.11. Coquinite beds of the Williamson Shale. A,B. absence su5cient Polished ccquinoid Limestone slabs from the WilLiamson suspension feeding. Vagrant epifauna, possibly Shale at LittIe WoIcott Creek (hc. 6). k Shows fining detritus feeders, are represented by calymenid upward with brachio~odcoquinite grading upwad into trilobites, rare starfish and Cyclonema gastro- laminated to cross-laminated calcisiltites; note prdomi- pods. The receptaculitid Ischadites was most nance of convex-downward orientations in concave-convex Eoplectodonta brachiopods. B. Displays well-preserved probably a photoautotrophic alga. Although Eopkctodonda and Atrypa brachiopods, note geopetal the dbnities of receptaculitids are still enigma- structures In shales. C. Polished coquinoid limestone slab tic, most recent studies of these fossils suggest of the Williamson Shale from Second Creek (Loc. 5) dis- that they were a type of heavily calcified algae plays a partly winnowed, dense accumulation of brachi* pcd shells; note spar filling areas between bmchiopads. (see Beadle and Johnson, 1986; Beadle, 1988 for recent reviews). chiopod coquinite appear to represent gutter (4) Inikpretution casts. The lithalogical charactwistics of olive gray Fine grained cross-laminated caIcisiEtites to medium bluish gray shales and moderately gradationally overlie some of the brachiopod- diverse benthic fauna suggest better bottom supported limestone layers (Fig.lL4, B). Coqui- oxygenation than in the deeper Monograptid- noid limestone beds often have sharp bases "'Chonetes" cornutus or Chondrites associa- (Fig.7E), and contain abundant greenish gray tions. Brachiopods display greater average size mudstone intraclasts (Fig.llC). Thin sections than those in the Monograptid-"Choneta" cornutus association. However, interfingering in dark greenish shales at Little Wofcott Creek with the latter association indicates a similar, Got. 6). In some cases only thicker portions probably adjacent, bathymetric position. of the bracbiopod valves remain calcifredf Predominant convex down brachiopod orien- atrypids from the upper Williamson show tations in shales suggests undisturbed burial decalcification of their thin frills whereas the in low energy environments (Appendix I). thicker portions of their shells remain calcified Articulated specimens of Liacalymene, ostra- but perhaps somewhat reduced in thickness. codes and other delicate fossils point to Molds of Atrypa frills display deformation and episodes of very rapid mud sedimentation. apparently underwent early dissolution prior The coquinites from Second Creek and Little to compaction of the muds. It is notable that Wolcott Creek closely resemble graded storm shells are commonly better preserved in black Iag deposits described by Kreisa (19811, Kreisa or dark gray than in green bioturbated shales. and Bambach (1982), &per (1982, 1985), and This is consistent with observations of shell others. Williamson coquinites display abun- dissolution in modern marine muds (Aller, dant evidence of episodic scouring of the 1982; Reaves, 1986). Fluctuating oxidation seafloor by storm waves and redeposition states within bioturbated muds of dysaerobic including sharply demarcated bases with sole settings enhance dissolution of carbonates. In masks, brachiopod valves with evidence of mud some modern environments iron sulfides pre- sheltering andlor sediment screening, and cipitated in near surface anoxic muds are upward gradation into laminated to cross- periodically oxidized (either by storm disturb- laminated siltstone with hummocky ripples. ances or bioturbation) forming sulfuric acid, Shell beds at Second Creek and Phoenix, also which lowers pH and causes carbonate dissolu- display elongated gutter casts on their bases tion (All=, 1982). resembling those described by Aigner (1982). The moderate diversity of benthic inverte- Coquinoid limestones and gutter casts in this brates in the Eoplectodonta-Atrypa associa- Williamson biofacies are interpreted as tem- tion suggests near-normal oxygen levels, at pestites resulting &om the combined action of the sediment-water interface. Occurrence of storm waves which scoured the seafloor and Dicoelosia and rare Skenidioides but absence of aggregated shells, and basinward directed Clorinda in the Eoplectodonta-Atrypa associa- gradient currents that deposited fine sands and tion indicates that this association belongs to silts transported from shoreward areas. an offshore benthic assemblage (BA-4 to BA-5, The great abundance of brachiopod shells in see Ziegler, 1965; Boucot, 1975). The coquinoid these coquinites as compared b background limestones suggest that, at least in part, the shales may reflect varied processes. The reIa- Eoplectodonta-Atsypa association lived above tively thick, persistent coquinites may record storm wave base, probably in less than about periods of low net deposition during which 50-60 m of water. shells grew and accumulated on the sedoor by The discovery of receptaculitids from the event condensation (cf. Fiirsich, 1982). How- Willowvale-Irondequoit interval is also of ever, the difference may also reflect tapho- considerable importance. Although the exact nomic bias; there is evidence for partial afl5nities of receptaculitids are debated, they solution of shells within greenish shales. probably represent a problematic calcareous Sepkoski (1978,1982) and Kreisa and Bambach algae (Nitecki, 1972; Beadle and Johnson, 1986; (1982) note that shells in aggregations are Beadle, 1988). [see Brett et al., in press for more better buffered against acidity and may be we11 detailed discussion of the problems involved in preserved, while those in carbonate-poor muds interpreting absolute depth from algae]. There- were subject to dissoIution. fore, their occurrence in the Eoplectodonta- Poorly preserved molds of the brachiopods Atrypa association may indicate that, at Ieast Atrypa, Eoplectodonta, and Coolinia are found parts of this association occurred in the in the Willowvale Shale indicates that this with Costistricklandia in the Willowvale Shale association belongs partly to Boucot's (1975) indicates that this association belongs partly BA-4. Much of the Willowvale Shale was to Boucot'a (1975) BA-4. deposited in moderately shallow marine en- vironment that supported a diverse fauna. This Eocoelia sukrmta-"Chonetes" cornutus facies should probably be assigned to BA-3 to 4 association: olive gray shale facies despite the lack of diagnostic brachiopods, because of the abundant bryozoans and by (1) Fossils analogy to Rochester Shale bryozoan beds Low diversity assemblages (richness= 16 (Brett, in press). As in the previously discussed 15 species, TableI), strongly dominated by Eoplectodonta-Atrype association, abundant brachiopods and bivalves, characterize Wil- winnowed coquinoid limestones occur within IowvaEe outcrops east of Utica area in eastern this association. This indicates that the Palaeo- N.Y. They also typify correlative beds in the cyclus-bryozoan association existed in water upper Willowvale Shale of Pennsylvania. Bra- depths of between normal and average storm chiopods form mono- or paucispecific assem- wave base (probably about 20-50m). It is blages on bedding planes includmg Eocoelia probably somewhat shallower than the Eoplec- sulcata, Chonetes cornutus, Atrypa reticularis todonta-Atrypa association as it contains a and Leptaena rhomboidalis. Poorly preserved greater abundance of coquinoid limestones bivalves (Ctenodonta, Cornellites, Leptoclesma, and, in eastern sections, more fine grained Pyrenomoeus) and the ostracodes Dibolbinrs sandstone and siltstone layers. It also lies and Mastigobolbina are abundant on many shoreward of the latter assemblage (Fig.l2$, bedding planes; others are barren or with only and offshore of the Eocoelia- "Chonetes" cornu- obscure burrows. Graptolites have not been tus association. Association of Palueocyclus found.

W E SE Rochester Syracuse Utica G,G S,M SF Ly P,H B,R Vp WIV 0,H V,H Y Normal Wave Base ------Average Storm Wave Base - - /BA-3 BA-2 y asrobfc I I I BA-4 I t dysaeroblc ? 1 I I

Eoplectodmca - At- I Palaeocyclus - I Eocoel La - I bryozoan I "Chonetes" - Chondrites - dysaerobic basin deeper ahelf temporary Inner shell ahoala mud8

Fig.12. Bathymetry and paleoecology of late Llandoverian (Silurian) benthic assemblages. GG= Genesee Gorge, Rochester, SM=Salmon Creek West, Williamson, SC = Second Creek, Alton, BR = Brewerton, Cicero, VR = Verona Station (Thruway cut), Verona, VH= Van Hornesville, Stark (2) Sedimentology and taphanomy which underlie the Willowvale in eastern New Fossils occur on bedding planes within York, grade into reddish marginal marine yellowish weathering olive to greenish gray sandstone (Otsquago facies) within a distance crumbly shdes. Most brachiopods are disarti- of about 15-30 km E of the Verona-WilIowvde culated and commonly fragmented. Shells typi- area of optimal development. AII of this cally occur in convex up pavements of single suggests that the Eocoelia su lcata- "Chonetes" valves. As in other beds of the Williamson- cornutus association existed only a few tens of Willowvale, shells display partial dissolution kilometers offshore. The background sedi- and are preserved as molds. ments consisted of muds indicating generally Thin beds of laminated fine-grained greenish low energy conditions apparently reflective of gray sandstone with sharply defined soles a quiet water nearshore belt ("2" zone of Irwin, displaying convex up shells and are inter- 1965). bedded with the shales. Thin sandstones and coquinites display Brachiopods, mainly Eocoelia, may occur in wave-formed ripples and evidence for multiple densely packed, lenticular coquinites 1-10 cm winnowing indicating episodic turbulence in maximum thickness. These beds display events on the seafloor. Low diversity-very high sharp bases and chaotic internal arrangement dominance associations of brachropods and of shells, such coquinite lenses are very small bivalves that typify thzs biofacies appear abundant in the equivalent upper beds (Masti- to reflect hardy, opportunistic species that gobolbina typus zone) of the Rose Hill Forma- existed in local dense populations in a stressed tion in Pennsylvania (Swartz and Swartz, 1931; shallow water environment. It is notable that Swartz, 1934) but rare in the equivalent one of the species, "Chonetes" cornutm, dso Willowvale in eastern New York. occurred in prolific assemblages in offshore areas stressed by oxygen deficient conditions. (3) Interpretation In the eastern Willowvale Formation and, This association is very simiIar to previously particularly in the Rose Hill Formation, evi- described Eocoelia communities (Ziegler, 1965; dence suggests that stress factors may have Ziegler et a]., 1968; Cocks and McKerrow, included high turbidity, episodic and rapid 1984). As with the latter, we infer that the deposition, and, perhaps, sIight fluctuations in eastern WilIowvale Shale and its counterpart salinity. This physically stressed environment in the upper few meters of the Rose Hi11 Shale apparentIy was unfavorable to most high level in Pennsylvania, were deposited in shallow, suspension feeders. Crinoids and bryozoans inner shelf muddy seafloers. Assignment to decline markedly in the transition from the benthic assemblage 2 seems reasonable; see Palaeocyclus-bryozoan association. In con- Berry and Boucot (1972). In New York State, trast, infaunal bivalves (Ctenodonta, Pyreno- the eastern shoreline facies are not seen moeus, Modiolopsis) were much more abundant because the Willowale Shale is truncated by a than in the western biofacies. Iater (late Silurian, Salinic?) erosion surface A curious anomaly of the Willowvale bio- beneath the Syracuse Formation (Rickard and facies is the near absence of typical BA-3 Zenger, 1964). However, in Pennsylvania, the pentamerid assemblages, at least as a '"high upper Rose Hill, bearing the Eocoelia sulcata- energy" or "shoal. type" fauna, Rather, the "Chonetes" association grades eastward into shales bearing Palaeocyclus-bryozoan bio- purplish shale with distinct coarsening upward facies appear to grade directly into seemingly sandstone cycIes; a few kilometers to the east, similar shales containing the BA-2 Eucoelia- this passes into the Lizard Creek Sandstone "Chonetes " cornutus associations. Member of the Shawangunk Formation The crinoidal, Palaeocyclus and bryozoan (Swartz and Swartz, 1931). However, Eocoelia bearing limestones appear to be a hint of the communities in the upper Sauquoit beds, high energy offshore belt. Also, the Willowvale and overlying Dawes Formation contain the associations appears to parallel increasing greatest frequency of sandstone beds near frequency of bottom disturbance by waves and Clinton, New York; while both units consist currents from deeper basinal areas to a higher- predominantly of shale in a eastern (shore- energy, shallow shelf setting; this is again ward) direction. This may indicate the pres- replaced, eastward, by low energy facies. In ence of a zone of wave dissipation ("Y" zone of general the biofacies appear to have been Lwin, 1965) offshore of low energy inner shelf elongate parallel belts along the onshore- muds @A-2-Eocoelia biofacies). offshore gradients of the WiIliarnson-Willow- vale sea. The monotonous green platy shales of Discussion the Chondrites association were deposited in low energy settings rarely disturbed by storm General bathymetric model for upper Clinton event deposition. These probably accumulated Group biofacies below deep storm wave base. The Monograp- tid-etChonete~'' comutus association of the Williamson-Willowvale fossil assemblages WilIiamson probably represents relatively can be subdividd into five recurring associa- deep water, normally low energy dysaerobic tions (Figs.12 and 13), as follows: (a) Chondrites settings, episodically influenced by storm in- association, @) Monograptid- "Chonetes" cor- duced gradient currents, about 50-80 m deep. nutus association, (c) Eoplectodonta-Abypa The more diverse Eoplectodonik-Atrypa asso- association, (d) Palaeocyclus-bryozoan asso- ciation intergrades with the Monograptid- ciation, and (e) Eocoelia su lcata- "Chonetes" "Chnetes" cornutus association but appar- cornutus association. The spectnun of fossil ently represents slightly better oxygenation, as evidenced by increased brachiopod size, famd diversity and the blueish to greenish gray coloration of shales. This environment was probably also slightly shallower, as indicated by a higher frequency of graded coquinites. These beds suggest occasional effects of deep storm waves directly on the bottom. In ad-

BRYOZOAN dition the occurrence of probable algal fossils (receptaculitids) probably demonstrates depo- sition in lit waters. The Palaeoqclus-bryozoan DfCOELOSlA association deveIoped in relatively shallow water frequently affected by storm waves and within the euphotic zone, Finally, the Emoelia suhta-'%honetes" association represents a shalIow water but relatively low energy inner sheIf mud belt. Fig.13. Biofacies and bathmetry of the Williamson- Willowvale interval. The Manograptid-"Chom&sS' and Comparison with other communities Chondriies associations represent deeper water condition the and form are similar to Vkbyella communities of the Williamson-Willowvale fossil assemblages Welsh Borderland (cf. McKerrow, 1978); the Eopkctw don&-Atrypa and PaIaemyclus-byozoan associations are contain many genera in common with other successively shallower water and closely resemble the Silurian assemblages and thus permit direct Dicoehia and Strickbdia communities of the Welsh comparison with previously described commu- Borderland. Evidence of storm-generated deposits in all nities. biofacies also suggests the waters were not very deep, instead they represent dysaerobic to fully aerobic settings, Studies of the Rochester Shale fauna (SiIu- with water depths of about 50-80 m, at the deepest. rim; WenIockian) and depositional environ- rnents in western New York (Brett, 1983) A Howellella-Leptaena-fenestrate bryozoan yielded six intergrading fossil associations as community was proposed for diverse assem- follows: (1) Chondrites, (2)Amphis t rophia-Da l- blages of bryozoan-rich pack- and grainstones manites, (3) Striispirifw, (4) Atrypa-bryozoan in the middle beds of the Rochester Shale; this patch, (5) encrinites, and (6) fistuliporoid was interpretd as a BA-3, moderately high bioherms. The Chondrites association is com- energy environment (Brett, in press). Storm- mon to both. Apparently, the Monograptid- generated calcarenites and cdcirudites of bryo- "Chonetes " association of the Williamson zoam in the Rochester Shale display mixtures Shale has no counterpart in the Rochester of well preserved and broken, abraded, and ShaIe and thus may record unique environ- corroded fossils indicating multiple episodes of ments more dysaerobic andlor deeper than reworking and condensation of proximal tem- those of the Rochester Shale. Conversely, the pestites; these are very similar in 1ithoIogy and Amphistrophia-Dalmanites and Striispirifer fauna to coquinites in the Palaeocyclus- associations of the Rochester are absent or bryozean association of the Willowvale Shale. poorly developed in the WilIiamson-Willow- Thus, portions of the Willowvale and the vale interval. These assemblages may have Williamson were deposited in environments been adapted to higher turbidity areas un- similar to those of the Rochester Shale, and, as represented in the Williamson. in that unit, may have accumulated partly Recently, Brett (in press) further subdividd above storm wave base. the five genedized Rochester Shde associa- Calef and Hancock (1974) studied WenIock- tions into a series of 12 comunities, several of ian and Ludlovian marine communities in which are loosely comparable to the William- Wales and the Welsh Borderland and described son-Willowvale associations described herein. a set of five major Silurian communities In particular, the broadly d&ed Atrypa offshore from the Lingula community, Salo- bryozoan patch association (Brett, 1983) has pina, Homeeospira / Sphaerirhynchia, Isorthis, been subdivided into the diverse ramose bryo- Dicoelosia, and the deepest water VisbyelIa zoandominated Atrypa-Dicoehia and Howel- community. The depth-related environmental Eella-Striispirifer communities, typical of lower factors determining the distribution of SiIu- Rochester shelIy mudstones in western New rian benthic marine communities probably York; brachiopoddominated Eopkctodonta- included substrate, water turbuIence and food Dicoelosia, and Whitfieldella-Dalejina cammu- suppIy. Increasing size of brachiopod lopho- nities occupy analog stratigraphic positions in phores with depth apparentIy reflects a limited central New York These are normally mud- food supply (Fiirsich and Hurst, 1974). supported skeIetal accumulations with minor Similarly, Hurst (1975) studied Wenlockian evidence for storm-winnowed Iayers interpreted carbonate, level bottom, brachiopoddomi- by Brett (in press) as belonging to BA-4 to BA-5. nated communities fiom Wales and the Welsh The strongly brachiopod-dominated Eoplec- Borderland and again recognized four basic todonta-Atma association of the Williamson benthic manine communities: Sphaerirhynchia Shale is very similar to the slightly higher wikoni (argillaceous micrite and biomicrite diversity Eoplectodonta-Dicoelosia community phases), Isorthis clivosu, Eopkctodonta duvalii, of the Iower Rochester Shale; all species of and Visbyelh treruerwa. He considered rates of brachiopods found in the former are also sedimentation, turbulence and substrate type present in the latter community and the to be primary factors controlling distribution dominance of Eopkctodonta and, in some beds, of brachiopods in shaIIow water environments. Dicoelosia is typical of both associations. Brett These British communities bear many sirni- (in press) considers the Eoplectodonta-Atrypa larities to those of the New York sequence. The association to be a subcommunity of the Monograptid- "Chonetes" cornuttls association Eoplectodonta-DicoeIosia community. of the Williamson Shale is somewhat simiIar to the VisbyelIa association in containing a (BA-3) occurs in subtidal areas between normal dominance of pelagic graptoifites and nauti- wave base and the lower limit of active loids. In both associations, the benthic fauna is photosynthesis (< 80m), BA-4 and BA-5 repre- dominated by small reclining brachiopods; sent areas within the subtidal zone and below common taxa include chonetids, and small the lower limit for active photosynthesis and specimens of Eoplectodonta and Atrypa. The reef-building ( < 150-200 m). nominal taxa of each association, however, do The Williamson-Willowvale fossil associa- not occur in the other. The Eopledodonta- tions appear to fall in Boucot's BA-2 to BA-5 Atrypa association of the Williamson-Willow- categories based on taxonomic comparison. vale interval shares many characteristics with Some portions of the Chondrites and Mono- the DicoeLosia communities of Fiirsich and graptid-"Chonetes" cornutus association bear Burst (1974) and the very similar Eoplectodonta comparison with the Visbyella community of duvalii community of Hurst (1975). These three the Welsh Borderland. Hence, these asso- associations share several closely related taxa ciations belong ta BA-5. The main William- including Eop kectodonta, Atupa, Dicoelosia, son brachiopod association (Eoplectodonta- and Coolinia. Similarities between the PaIaeo- Atrypa) is clearly related to the British Dime- cyclus-bryozoan association and the Isorthis losia or Clorinda communities, and in the case communities iilclude high diversity of brachio- of the Willowvale there are hints of the pods and abundant crinoid and bryozoan Costistricklandia community. Hence, all Wil- debris. liamson biofacies reflect offshore communities, while the Willowvale includes the shallow Implications of late Llandoverian benthic shelf Palaeocyclus-bryozoan and nearshore assemblage model Eocoelia associations. The relative onshore-offshore order of these Ziegler et al. (1968) studied the composition communities or benthic assemblages, first rec- and structure of Early Silurian (Llandoveriaa) ognized by Ziegler (1965), is strongly corrobo- marine communities and described five benthic rated by the resulb of this study of New York brachiopod communities in detail: (1) Lingula, Llandoverian fossil associations; there does (2) Eocoelia, (3) Pentamerus, (4) Strickkandia, appear to be a bathymetically related pattern, (5) Clorilada. These communities occupied shelf at least in the distribution of Williamson- regions of the British Isles, Norway, and North WilIowvale brachiopod associatiam. In that America in the late Llandoverian, and are any given brachiopod association may occur in thought to represent a spec- of marine varied lithologies, from siliciclastic to carboa- environments ranging from the coast to the ate, there appears to be less dependence on limit of the shelf (Johnson, 1987). Subsequent substrate composition. In the case of bryozoans studies of Silurian fossil communities in and pelmatozoans, however, there does appear Britain and elsewhere have confirmed this to be an association with more caIcareous general pattern, though the paleoecologica~ facies, while bivalves clearly predominate in interpretations differ (Ziegler et al., 1968; Calef silty mudstones and siltstones and are much and Hancock, 1974; Fkich and Hurst, 1974; less abundant in calcareous facies. As noted Hurst, 1975; see Boucot, 1975; Cocks and above, sedimentological evidence suggests that McKerrow, 1984, for a review). none of these facies were deposited in ex- In summarizing earlier Silurian studies, tremely deep water. Boucot (1975) classified Silurian brachiopod It is possible to place some constraints on the communities into five "benthic assemblages'" absolute depths of benthic assemblages 4 and 5, (BA-1 to BA-5) corresponding to Ziegler's as they occur in the Llandoverian of New original broadly dehed "commmities". As York. If these assembIages are to be based interpreted by Boucot, benthic assemblage 3 primarily on taxonomic composition, there appear to be some fundamental problems with interpreted as a shallow water zone approxi- the bathymetric zonation, as proposed by mately coinciding with normal or average Boucot (1975). Boucot interpreted the BA-3 to storm wave base. Hence, BA-3 indicates a BA-4 boundary as the base of the euphotic commonly high energy zone affected fre- zone. However, the occurxence of Ischadites in quentiy by wave action. Assuming, as does the Eopleetodonta-Atrypa association of the Boucot (19751, that DieoelosiQ and Clorinda Williamson and in a Cbrinde-dominated as- associations belong to the transition zone of semblage from the overlying Irondequoit For- BA-4 to 5, this transition could be interpreted mation raises a possible problem for this as approximately equivalent to the base of the interpretation. The receptaculitid fossils are euphotic zone. BA-4 would also represent well preserved and almost certainly were not environments affected by scouring of deep transported into these assemblages. Unless storm waves. BA-5 could be restricted to previous interpretations of receptaculitids as Vis by ella-type communities, which were per- photoautotrophic algae are incorrect, these haps deposited belbw the photic zone and near occurrences indicate euphotic conditions for the lower limit of deep storm wave effects. classic BA-4 to 5 associations (Clorinda, Dicoe- Because communities were controTled by many iosia). Much stronger evidence for a photic more factors than bathymetry, the benthic zone position of BA-4 assemblages is found assemblage boundaries may not coincide in an elsewhere Ce.g. Norway) where cyclocrinitids, absoIute way with water depth. However, the almost certainly a type of dasycIadacean dgae, model proposed here provides at least a first occur with stricklandiid communities (Beadle, approximation of the depth zonation of benthic 1988). Furthermore, abundant evidence for assemblages. episodic storm wave impingement on the seafloor occurs within the Eoplectodonta- Acknowledgements Atrypa association and, especially the Palaeo- cyclus-bryozoan association. This evidence This paper is based on part of an M.Sc. thesis includes occurrence of winnowed, sometimes done at the University of Rochester, graded shell beds, hummocky cross lamination, Rochester, N.Y. The authors wish to thank Dr. and erosional gutter casts. D. V. Ager and Dr. A. J. Boucot for review of Laminated fine-grained sandstones of the the submitted versions of the manuscript. J. Monograptid-"'Cho~te~es" association may have Eckert provided valuable assistance during been deposited below storm wave base, perhaps field work and photography. by gradient currents. This indicates that maxi- mum storm wave base may have coincided Appendix I - Fauna census of the approximately with the BA-4 to 5 boundary. Williamson Shale from Little Wolcott Creek This evidence accords well with the alga1 and Second Creek evidence that these assemblages may represent approximately 20-80m depths. In turn, this range is substantially shallower than earlier I. Little Wohtt Creek (1) Dark greenish shale Disarticulated Articulated estimates of up to 200 rn for analogous fossil (5GY 411) Brachiopod convex convex assemblages. Based on evidence as. mentioned up down above, the following revisions for benthic At~ypurelialaris 4 14 0 assemblage bathymetric zonation have been Eopkctodonta iranswrsalis 10 73 2 proposed. Coodinia subplana 12 0 The BA-3 to BA-4 boundary as defined TotaI 16 89 2 taxonomically (e-g. Penturnerus to Costistrick- Ratio for (disarticulated/articulated)- 52.0 landia association) does not correspond to the Ratio for (disarhculated convex downftotal base of the euphotic zone,but rather should be disarticulated)= 891'104 = 0.856 County, New York (LJSGS, Rochester West 7.5' Quad- (2) Coquimid limestone Disarticulated Articulated rangle). Brachiopod convex convex (2) Genesee Gorge East: A. Rocks exposed in south cliff up down at Maplewood Park, eastern side of Genesee River Gorge, near St. Paul BIvd., north of Scranton Street, about 2&50 rn difF formed by Rochester Shale and Irondequoit Limestone, Roch~ter,Monroe County, New York, (USGS, Rochester East 7.5' Quadrangle). B. Rocks exposed in east Total 125 16 cliff, approximately 150 m south of Seth Green Drive, Ratio for {disarticulaM/artinrlated) = 125116 = 7.81 Rochester Gas Electric Access Road Rochester, Monroe Ratio for (disarticdatd convex downjtotal County, New York (USGS, Rochester East 7.5' Quad- disarticulated) = 701125 = 0.56 rangle). (3) Tryon Park (Ether's Glen): Palmer's Glen, exposure XX. Second Creek on north side of small tributaq stream flowing ~ntowest (I) Light olive gray shale (5Y 611) (unit 36) and [greenish side of Irondequoit Bay, just east of Route 1-590 about 1km gmy shale (5GY 5/11 (unit 37M north of Browncroft Blvd., and 150 Ism south of Tryon Park Atsypra reticular~s 1 1 Road, Rochester, Manroe County, New York (USGS, Eoplectodonta transwrsalk 29[1] [14] Rochester East 7.5' Quadrangle). Coo linia subplana 2 I4 2[31 (4) Salmon Creek West: Rock exposure along west Salmon starts 104 and Total 33 17 6 Creek 60 m south of Route continue intermittently to the railroad bridge, Town of Williamson, *LiocaLymne clintoni 1ill Wayne County, New York WSGS, Williamson 7.5' Quad- **a few pyritized Monograptus clintonensis rangle). (5) Second Creek Williamson Shale exposure is located Ratio for (disarticulated/articuIated)= 5016 = 8.33 at 2250 m north of the bridge on Route 104 over Second Ratio far (disarticulated convex downltotal Creek and extends from I00 to 400 rn south of Red MiLl disarticdatd) = 17/50 = 0.34 Road, Hamlet of Alton, Town of Sodus, Wayne County, York Rose (21 Coquinoid limestones (unit 38) New (USGS, 7.5' Quadrangle). Atr-ypa reticubrts and (6) Little Wolcott Creek: Small bank Williamson Shale Eopkctodonih tmmversalis 34 56 4 exposure on Little Wolcott Creek, approximately 300 m south of East Port Bay Road, Town of Wolcott, Wayne Total 90 4 County, New York WSGS Wolcott 7.5' Quadrangle). (7) Phoenix: Ratio for (disartieulatedjarticulated)= 9014 = 22.5 Dumps of WilLiamson Shale dong northeast Ratio for (disartieulated convex downltotal banks of Oswego River, 950 m south of Route 57 and 3M3 m disarticulated) = 56j90-0.62 northwest of Town Line Road, Great Bear Springs, Town of Schroeppel, Oswego County, New York (USGS Pennell- (3) Medium blueish gray (5B 5/11 (unit 39) vilIe 7.5' Quadrangle). Alrypa reticularis 33 14 (8) Brewerton: Based on report of GilIette (1943,one of Eop bctodonia transversalis 332 26 the best outcrops for collecfmg fossils is on United States Coolina subplana 27 Highway 11, approximately 150 m south of Oneida River, Leptaena rhomboidalis 3 Town of Cicero, Onondaga County, New York. At present PhoLidops squummiformis 2 the outcrops are destroyed. Total 397 40 (9) Verona Station: Low ditch exposures along inter- change no. 33 exit ramp from NY State Thruway onto *Liocolymem clintoni 3 (1 cephalon I Route 365, 350 m south of Thruway (1-90). 0.5 km east of and 2 molds) Route 365, Verona Oneida County, New York (USGS, **Asternidea (Paleaster ? sp.) I Vernon 7.5 Quadrangle). *)*a few MastigoboEbim pumtuta (10) Lairdsvllle: Exposures of Williamson Shale in banks of small north-flowing tributary of major east-flowing Ratio for (di~cula~/articulated= 397140 = 9.925 tributary of Oriskany Creek 5&200 rn north of NY Route 5 (Senem Turnpike) about 1.0 km east of hamlet of Lairdsville, Town of Westmoreland, Oneida County, New Appendix I1 - Locality register (in order York (USGS, Clinton 3.5' Quadrangle). (11) Dawes Quarry Creek: Exposures of Willowvale from west to east) Shale on small tributary on north side of Sherman Brook (locally called Dawes Quarry Creek) about 500 rn southeast (1) Genesee Gorge West: Rocks e~pasedin north cliff of of bridge of New Road over Dawes Quarry Creek, Village Maplewd Park, western side of Genesee River Gorge. of Clinton, Town of Kirkland, Oneida County, New York Follow path north of Park about 50-IM3 rn on terrace (USGS,Utica West 7.5' Quadrangle). formed by Reynales Limestone, Rochester, Monroe (12) Willowvale Glen: The Glen, east-flowing tributary of Sauquoit Creek at Willowale, 2.5 krn west of Bridgewater Boucot, A. J., 1975. Evolution and Extinction Rate St., 1 km northwest of Chadwicks, Town of New Hartford, Controls. Elsevier, Amsterdam, New York, 427 pp. Oneida County, New York WSGS, Utica West 7.5' Boucot, A. J., 1985. The relevance of biogeography to Quadrangle). palaeogeographical reconstructions. Phil. Trans. R. Soc. 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