B4: PALEOENVIRONMENTAL GRADIENTS ALONG A RAMP TO BASIN TRANSITION IN THE
MIDDLE DEVONIAN LUDLOWVILLE FORMATION OF CENTRAL NEW YORK STATE
Carlton E. Brett
Department of Geology
University of Cincinnati
Cincinnati, Ohio 45221-0013
Gordon C. Baird Department of Geosciences
SUNY College at Fredonia
Fredonia, NY 14063
Alex J. Bartholomew Department of Geological Sciences
SUNY College at New Paltz
New Paltz, NY 12561
INTRODUCTION
The richly fossiliferous Middle Devonian Hamilton Group strata of central New York State form a natural laboratory for the study of paleoecology and the relationship of organisms to paleoenvironmental changes during cycles of varying magnitude. Indeed, these rocks have formed the basis for numerous detailed studies about paleoecology (including; Brower, 1987; Bower and Nye, 1991; Brett et al., 1990). These studies have focused largely on vertical gradients of faunal replacement and make a tacit Waltherian assumption that the vertical relationships seen in medium and small-scale cycles within the Hamilton Group reflect lateral changes; however, it is rarely possible to actually test this assumption. Much of this recent work has been framed in the context of sequence stratigraphy and the presence of small-scale sequences and cycles in central New York State this forms part of the basis for our somewhat revised model of siliciclastic-carbonate depositional sequences in foreland basins (see Brett and Baird, 1996).
The Middle Devonian Ludlowville Formation in the eastern Finger Lakes region provides a particularly excellent opportunity for examination of these issues (Brett et al., 1986). The rocks are relatively simple structurally, and are well-exposed in numerous creeks and shoreline bluffs along Owasco, Skaneateles, and Otisco lakes (Owasco, Skaneateles, and Spafford 7.5'quadrangles). These lakes, with relatively northwest-southeast orientation oblique to facies strike, provide natural cross-sections across rapid facies changes particularly within units of the lower Ludlowville Formation. Those units, comprising the Chenango - Centerfield succession and the overlying Otisco Member, are the subject of the present study. In particular, we focus upon the lowest 2-3 cycles of the Ludlowville Formation, consider their sequence stratigraphic context and mode of origin, and describe an unusually steepened gradient within one of these units, the Stag Horn Point submember of the Otisco Member. Together, these units provide insights into both vertical and lateral changes in benthic marine fossil assemblages that are a response to relative water depth as well as variations in sedimentation. We begin with an overview of the physical and sequence stratigraphy of the Chenango to lower Otisco interval and then focus on the details of the Stag Horn submember in the region of Skaneateles and Otisco lakes. The result of this study is a fairly detailed description of lateral as well as vertical gradients of faunal change during the deposition of this interval.
CONCLUSIONS
The middle portion of the Hamilton Group in the eastern Finger Lakes area provides an excellent case study in the interactions of relative sea level fluctuation, related changing patterns of sedimentation, and concurrent biotic responses. Decameter and smaller scale sedimentary cycles are superimposed on a larger scale pattern of sea-level rise and fall, represented by the upper Skaneateles Formation and the Centerfield and Ledyard/Otisco members of the Ludlowville Formation.
Sea-level highstands in this area of the foreland basin were characterized by low diversity assemblages of brachiopods and small mollusks, adapted to dysoxic mud substrates. Falls in relative sea level not only led to shallowing of the seafloor but also increased progradation of muds and silts into the basin. Faunas changed upward to moderate diversity assemblages of larger semi-infaunal brachiopods and bivalves, as well as the deposit feeding organisms responsible for the complex spreiten trace Zoophycos.
Conversely, rises in relative sea level were associated with abrupt reduction in sediment supply. Concretions and phosphatic nodules may have developed in older sediments during an interval of sediment starvation representing rapid sea level rise. Highly diverse assemblages including, but not limited to, abundant rugose and some tabulate corals, occupied the shallowest water settings during relatively clean siliciclastic free intervals. In the case of the lower Otisco Stag Horn cycle, a thriving coral biostrome was developed on a shallow prograded siltstone platform during this phase. This example also shows that during times of relatively low sediment input, normal submarine processes such as storm-generated waves and currents may be effective in eroding previously deposited sediments and transporting them further basinward, particularly along the sloping edge of a prograded platform.
The rapid lateral facies changes and constrained stratigraphy of the lower Ludlowville Formation in central New York provide excellent opportunities to examine lateral-, depth- and sedimentation-related gradients of environmental and biofacies change. This is an area wherein exceptional preservation and exposure can be used to test general hypotheses about the connection between physical environmental factors and biotic change on the scale of millennia to hundreds of thousands of years.

B4: PALEOENVIRONMENTAL GRADIENTS ALONG A RAMP TO BASIN TRANSITION IN THE
MIDDLE DEVONIAN LUDLOWVILLE FORMATION OF CENTRAL NEW YORK STATE

Outcrops of Ithaca Fm.

View of bedded kame gravels to left

View of bedded kame(?) gravels to left

High vista of Skaneateles Lake ahead

Old quarry to left in Tully Limestone

Contact of Fall Brook and Fisher Gully submembers
Contact of Fall Brook and Fisher Gully submembers of the Windom Member of the Moscow Fm. in ditch

View of Skaneateles Lake

Glen Haven

Outcrop of Ivy Point Member siltstone

Cross gully. View of Spafford Landing cliffs to east across lake

Owasco / Tichenor contact
Contact between the Owasco Member of the Ludlowville Fm. and the Tichenor Member of the Moscow Fm.

Roadside shale pit in lower Windom Shale
Roadside shale pit in lower Windom Shale rich in Ambocoelia; Bay View bed rich in brachiopods and small corals is exposed in floor adjacent to road; higher banks are in the Bear Swamp submember of the Windom.

Three Mile Point ravine
Exposures of middle and upper Windom to west of road

Upper end of Glen Cove ravine

OPTIONAL STOP 1. Glen Cove Creek
If water levels in the creek are not too high we may proceed on foot to the mouth of Glen Cove Creek behind the marina. Beds exposed near the creek mouth are in the lower Otisco slightly above Moonshine Falls phosphate bed; these shales are rich in the small brachiopod Arcuaminetes scitulus and carry small concretions and pyritic nodules. Upstream a few hundred feet the Stag Horn Coral horizon is exposed near the base of a small falls. The bed here yields primarily Athyris and other brachiopods and small corals. Occasional corroded fragments of larger rugose corals may be obtained.
Hiking upstream and climbing the relatively gentle small waterfall provides access the Glen Cove shell bed (type section). Here the bed is rich in Athyris, the small rugosan Stereolasma, the bivalve Cypricardella and Modiomorpha among others. These exposures provide a view of relatively fossiliferous down ramp facies of Otisco shelly horizons.
NOTE: if weather permits we will take a boat trip from Sevy's Marina across Skaneateles Lake to the vicinity of Stag Horn Point and Jenney Point; see descriptions under driving directions for STOPS 1 and 2. If so, we will be on the boat for approximately an hour.

STOP 2. Stag Horn Point
Park in pull-off to left at bend; remainder of road is too steep for most non-four wheel drive vehicles; so proceed down to shore of Skaneateles Lake (∼0.25 mi.), passing by exposures of upper Ivy Point Member, Spafford, and Portland Point members in Chase Point Ravine; note exposures of Otisco Shale on steep road down to shore; past the mouth of Barber Creek proceed past the first cottage (permission for access from homeowners is required); bear left (southeast) and onto bench formed by Stag Horn siltstone. (Alternatively, this section may be accessed by boat).
Here the upper beds of the siltstone are exposed and show excellent spreiten of Zoophycos; occasional phosphatic nodules and corroded corals are present in the top of the silt. The bench is abruptly overlain by about 1.5 m of the Stag Horn Point coral biostrome; densely packed solitary rugosans, including Heliophyllum, Cystiphylloides, and Siphonophrentis, the latter up to about 50 cm long. Corals range from corroded flattened fragments to complete and in situ uncorroded skeletons; they are packed in a mudstone matrix; upper part of the bank shows normal middle Otisco Shale. The coral biostrome continues to the south for nearly a kilometer before dipping below lake level.

STOP 3. Coves SE of Jenney Point: Edge of Stag Horn Platform
Jenney Point, find parking (permission for access from homeowners is required); proceed, if possible, southwestward along shoreline. (Alternatively, this section may be accessed by boat).
The first cove southeast of Jenney Point displays the lower Otisco Shale at about the level of the Staghorn biostrome. Depending upon lake level it may be possible to walk along the shore to the next point and into the next cove to examine features of the transition of the platform edge into ramp sediments. Corals are present here in lenticular, wedge-shaped masses and layers that abut the truncated edge of the siltstone platform. Adjacent to the first cottage a large mass of rugose coral corals dips to the south about 8-10° along an apparent erosional surface. At the south end of this cove, the siltstone platform remains partially truncated; about 30m further to the north, near the middle of the cove, the pyrite- cored concretions, which normally lie below the platform are directly overlain by coral rubble. In places, the coral rubble thickens to about one meter as the truncation surface runs downward from the pyritic concretions to nearly the level of Skaneateles Lake.
The next cove to the south between an unnamed point and Chase Point, north of Stag Horn Point exposes a similarly intriguing series of beds including a series of thin graded beds of siltstone, locally with corals in their bases that appear to be storm event beds of silt eroded from the truncated platform margin (see text and Figures 6-8 for details). Together, these two cove sections provide a view of an eroded platform edge that was shedding coral debris and silt layers into the surrounding ramp area.

Borodino mud mound (micritic bioherm) in Tully Fm

Upper Windom Shale outcrops

Chenango Siltstone outcrop

Sharp bend, view of Otisco Lake

Road follows along Otisco Lake shore

STOP 4. Road Cut Along Coon Hill Road, Marietta, NY
This long cut exposes the complete transition from the upper Skaneateles Formation to the base of the Ludlowville Formation. Lower portions of the roadcut expose black, silty Butternut Shale. These shales yield a low diversity assemblage of Eumetabolotoechia. Its basal contact is exposed in a stream on the south side of the road. The upper 10 meters of the unit contains thin 1 to 2 cm tabular, laminated siltstone beds. The Butternut Shale is abruptly overlain by the Peppermill Gulf Bed, a 30 cm interval of medium gray calcareous and highly fossiliferous mudstone at the base of the Chenango Member.
From this point the Chenango Member, ∼10 meters thick, shows an upward-coarsening succession from shale and silty mudstone to Zoophycos-churned siltstone. Fossils are generally scattered but the brachiopod Tropidoleptus is common at many levels and an in situ cluster of the large brachiopod Spinocyrtia was found about midway through the succession.
Near the upper end of the roadcut the upper Chenango weathers as a distinct bench. Large favositid and rugose corals present in the upper calcareous sands and overlying thin, silty limestones. These beds signify the basal transgressive systems tract of the Ludlowville Formation. The Halls Landing beds (upper Centerfield Member), mudstones with thin shell rich horizons form the uppermost unit of the cut.
Uppermost exposures in the adjacent stream bank opposite the upper part of Coon Hill Road show the lower Otisco Shale. The Stag Horn siltstone bench is absent at this section and its position is only suggested by beds of silty shale rich in Athyris brachiopods.

STOP 5.(Optional)Shale Pit at Rose Hill
This shale pit exposes richly fossiliferous beds of the middle-upper portion of the Otisco Member. Shales contain diverse brachiopods, bivalves and other fossils and a horizon with scattered rugose corals toward the upper part of the pit appears to record the Joshua Coral Biostrome level, although it is poorly developed compared with exposures to the east on Otisco Lake. This section provides an excellent opportunity to collect fossils from the Otisco beds.

Village of Clintonville; red clay banks

Hogsback Rd. and Peppermill Gulf
Type locality of Peppermill Gulf Bed at base of Chenango Siltstone.

Tully Valley, Village of Cardiff, site of famous Cardiff Giant hoax

Large cut of Butternut Member of the Skaneateles Fm.

Windom Shale outcrops

Uppermost Windom Shale
Note large exposure of uppermost Windom Shale behind Best Western Inn to east of highway

Roadcuts in Ithaca Formation