B2: SILURIAN SEQUENCE STRATIGRAPHY, EVENTS, AND PALEOENVIRONMENTS ALONG THE CRATONIC MARGIN OF THE APPALACHIAN FORELAND
Carlton E. Brett
Department of Geology
University of Cincinnati
Cincinnati, OH 45221-0013
(carlton.brett@uc.edu)
Patrick I. Mclaughlin
Geology Department
Bucknell University
Lewisburg, PA 17837
Robert Jacobi
876 NSC
University at Buffalo
Buffalo, NY 14260
INTRODUCTION
Silurian rocks of the Appalachian Basin and the North American mid-continent platform provide an excellent suite of strata for application of sequence, and event stratigraphic approaches. The strata are well exposed and display marked vertical changes in facies, commonly associated with distinctive condensed beds and/or discontinuities. The Niagara Escarpment in western New York and the Niagara Peninsula of Ontario represents a key reference area for the North American Silurian System. Indeed, the term "Niagaran," which has been variably applied to the middle or lower and middle portions of the Silurian, is still commonly used in North America. These rocks have been well documented by many researchers, beginning in the mid 1800s (Hall, 1852; Grabau, 1901; Foerste, 1906, 1935; Williams, 1919; Gillette, 1947; Bolton, 1957; Zenger, 1965, 1971; Sanford, 1969; Martini, 1971; Rickard, 1975; Duke, 1987; Duke and Brusse, 1987; Duke and Fawsett, 1987; Brett, 1983 a, b, 1985; Brett et al., 1990, 1995, 1998; Goodman and Brett, 1994; LoDuca and Brett, 1994).
Previous reports on the Silurian in the Niagara region have focused on identification of depositional sequences and depositional/faunal events. A theme of this guide is the documentation and interpretation of Silurian facies, events, and sequences along the northwestern rim of the Appalachian Basin. As such, the discussion is not restricted to the sequences and facies of western New York. A remaining research agenda is to trace these sequences into other regions, especially the mid-continent. Ongoing research along the Cincinnati Arch, in Ohio, Indiana, and Kentucky has revealed striking similarities of sequences and events among these seemingly disparate areas, some 500 to 700 km southwest of the main Niagara Escarpment (Fig. 1). In this paper we review and update information on depositional sequences and events in western New York and Ontario and then discuss their lateral relationships with those of the Cincinnati Arch. A companion field guide will highlight the Silurian strata of the Cincinnati Arch region. The large-scale ("third order") sequences described herein are sharply bounded stratal packages, ranging from less than a meter (where partially truncated or condensed) to about 50 m in thickness. In each case, the sharp basal contacts, when traced in an up-ramp direction, change from facies dislocations (abrupt shallow over deeper facies change) to erosional surfaces that demonstrably truncate underlying strata. Most sequences recognized herein, display a generally deepening-shallowing pattern.
However, in detail, the larger-scale sequences are composites, divisible into smaller, fourth-order sequences.
With its renewed emphasis on through-going discontinuities and condensed beds, the sequence stratigraphic approach, has encouraged a broader, more regional view of stratigraphy, and an attempt to understand the genetic significance of particular beds and surfaces. To some degree it vindicates the earlier "layer cake stratigraphy" approach. Sequence stratigraphy, originally developed from remote seismic studies of passive margin sediment wedges (Vail et al., 1977, 1991; Wilgus et al., 1988; Coe, 2003; Catuneanu, 2002), is now being applied at the outcrop scale to diverse depositional settings such as the Taconic foreland basin (Brett et al., 1990 a, b; Witzke et al., 1996; Brett et al., 1998; Brett and Ray, 2006). Many distinct horizons in the local stratigraphic record are interpretable as sequence boundaries, drowning surfaces, and forced regression surfaces. Moreover, a number of phenomena, which occur non-randomly in the geologic record, from phosphatic nodule horizons to reefs, fit in predictable ways into depositional sequences.
A second theme of this article is the recognition of widespread events, such as storm deposits, (tempestites), rapidly buried fossil horizons (obrution deposits), and even seismically deformed beds (seismites). The Silurian Period has typically been considered a tectonically quiescent time; however, growing evidence from eastern North America suggests the Silurian was more dynamic then previously considered. For example, sedimentology and stratigraphic mapping of Silurian deformed beds in eastern North America demonstrate that these event beds are extremely widespread and that their component sediment layers were not deformed during initial deposition, but slightly later, during shallow burial, likely the result of shaking induced by large-scale earthquakes (McLaughlin and Brett, 2006). The distribution of deformed beds, together with increased subsidence, clastic influx, and K-bentonite horizons, provides a meter of intensity and timing of pulses of Silurian orogenesis. A variety of such catastrophic events is recorded in the Silurian rocks of the Niagara region and will be examined on this trip.

B2: SILURIAN SEQUENCE STRATIGRAPHY, EVENTS, AND PALEOENVIRONMENTS ALONG THE CRATONIC MARGIN OF THE APPALACHIAN FORELAND

Northern edge of ridges of Barre Moraine
Northern edge of ridges of Barre Moraine and possibly the Albion Moraine, which converges with it here.

Cross Erie/NYS Barge Canal.

STOP A. (OPTIONAL) Crain Street Quarry and Outwater Park
(brief stop).
View off Niagara Escarpment; spillways (occupied by East and West Jackson Roads) of ancient Lake Tonawanda into preglacial Lake Iroquois. Note Lake Ontario (about 18 km to north).
Walk back from overlook into old quarry at brink of escarpment to observed glacially polished surface of Pekin Member, Gasport Formation with small bioherms with well preserved stromatoporoids, favositids, and colonial rugose corals; pelmatozoan holdfasts are attached directly to bioherm surfaces. Patches of cross-bedded reef-flank debris are present at margins of bioherms.

STOP 1. Road Cut on Gooding Street
Below Somerset Railroad Viaduct (Queenston Shale-Medina Group)
This excellent outcrop has been described previously in considerable detail (see Friedman, 1982; Duke et al., 1987). It provides an outstanding exposure of the basal Silurian Cherokee Unconformity and a good opportunity to study the lower units of the Medina Group as well as the uppermost beds of the Queenston Shale.
About 6 m (20') of upper red mudstones and siltstones of the Queenston Formation (upper Ordovician. Ashgillian) are exposed at this locality. The Queenston has been interpreted as either very shallow marginal marine or non-marine red beds. The Cherokee unconformity in this area is the basal surface of Whirlpool Sandstone, which is nearly planar. It is also a megasequence boundary separating the early or Creek phase of Sloss' (1963) Tippecanoe megasequence from the later Tutelo phase see Dennison, (1989).
The Medina Group (Sequence I of the Silurian System), consists of an Early Silurian (early Llandovery, Rhuddanian), siliciclastic wedge derived from tectonic source areas to the southeast (Figs. 5, 21). The lowest Silurian unit, white Whirlpool sandstone is about 3.5 m (11.5') thick at this location. Basal beds of the Whirlpool Sandstone are quartz arenites with northwest dipping cross strata, which have been interpreted as non-marine, braided stream deposits (Middleton et al., 1987). Large-scale channel-like structures occur in, or at least at the top of, these sands. Shale drapes within such channels at Lockport have yielded marine acritarchs (M. Miller, unpublished data) indicating that the channels were backfilled by very shallow marine sands and minor muds during a rise of sea level. Hence the irregular channeled surface that separates lower Whirlpool braided fluvial facies from upper Whirlpool hummocky cross-stratified, sparsely fossiliferous beds is a transgress surface. The Whirlpool Sandstone thus is interpreted to contain either a lowstand (or shelf margin) systems tract and a transgressive deposit. A thin bed containing phosphatic pebbles and fossil grains occur with the Whirlpool-Power Glen contact. This phosphatic pebble bed may mark a marine flooding surface, or surface of maximum starvation associated with relatively increased rates of sea level rise. This surface marks the change from shallow shelf sands of the upper Whirlpool into deeper shelf muds and storm sands of the upper Whirlpool into deeper shelf muds and storm sands of the Power Glen Formation, herein interpreted as highstand deposits. The Power Glen exhibits subtle small-scale parasequences.
This outcrop is one of the easternmost exposures of the Power Glen Shale. At this locality the Power Glen Shale comprises of about 5 m (16') of greenish gray shale with thin tempestitic siltstone and sandstone beds. The basal meter-thick transitional zone consists of thin (2-10 cm) muddy sandstones with interbedded sandy shales. Sandstone in the Power Glen Shale feature small-scale hummocky lamination and gutter casts suggestive of shallow, storm influenced shelf deposition. Small burrows (Planolites) are common, but body fossils are rare.
Greenish to reddish sandy shales and reddish sandstones occur near the top of the Power Glen suggesting a minor upward shallowing trend. However, the top of the unit (as defined herein) is sharply demarcated at the base of a massive white to pink mottled sublitharenitic sandstone about 2.5 meters (7.7') thick, the Devil's Hole Sandstone (Brett et al. 1995) seen at Niagara Gorge (Stop 6). The basal and upper beds of the sandstone contain lingulid brachiopods and probable Lingula burrows.
The white sandstone appears to record relative sea-level drops during which sands were distributed widely into the basin. The unit has some characteristics in common with the upper member of the Whirlpool Sandstone and, by analogy, is considered to be a relative lowstand to transgressive deposit.
The unnamed sandstone, in turn, is overlain by about 2.0 meters of brick red shales and interbedded sandstones assignable to the lower Grimsby Formation. These beds are ferruginous and exceedingly rich in fragments of lingulids with rare nautiloids and bryozoans. Thin spastolithic (oolitic) hematite stringers occur near the top of the unnamed sandstone and probably reflect reworking of sediments in shallow marine environments during an interval of sediment starvation. Hence, these shell-rich ferruginous sediments represent a condensed interval at the base of the Grimsby highstand deposits.
The reddish marine shales near the base of the Grimsby Formation pass upward into red and white-mottled sandstones and thin sandy shales. These beds are exposed high in the cut and are not readily accessible. This upper interval will be seen to better advantage at Niagara Gorge (Stops 4,6).

STOP 2A. Lockport Junction Road Cut (Lower)
Upper Medina and Lower Clinton Groups
This cut along Lockport Junction Road exposes upper units of the Medina Group and the lower part of the Clinton Group. The lowest beds are exposed beneath and just north of the overpass of Lower Mountain Road over Route 93. The basal units seen here are red shales near the top of the Grimsby Formation. These shales are overlain by 1.0 to 1.2 meter thick, blocky, pinkish gray sandstone that displays color mottling due to bioturbation. Swirly spreiten of the trace fossil Daedalus occur sporadically near the top of the sandstone ledge. Detailed regional correlation by Duke and Fawcett (1987) indicates that this unit is the equivalent of the Thorold Sandstone at the Niagara Gorge. The upper contact of the sandstone is marked by a thin red silty bed containing a "hash" of lingulid shell fragments.
The Thorold, in turn, is overlain by 1.7 to 1.8 m of dominantly red silty shale of the (Cambria Member) uppermost Medina Group. This shale bears distinctive fine, white mottling due to bioturbation. Although previously assigned to the Grimsby Formation, this is a distinctive, ostracode-bearing shaly unit that overlies the Thorold Sandstone and is traceable regionally at least as far as east as the Rochester area. Brett et al. (1995) termed this unit Cambria Shale Member of the Thorold Formation, using this section as the type locality. The upper portion of the Cambria Member is pale purplish to greenish gray sandstone and sandy shale that was formerly termed Thorold Sandstone. In actually, this is simply a leached sandy zone in the Cambria Shale. The Kodak Sandstone and about 1 m of Cambria Shale have been removed here at the sequence I/II unconformity. Sandstones contain small Skolithos burrows and intercalated green shale beds, especially the topmost layer, contain prolific leperditiid ostracodes. The greenish color extends down about 30 to 50 centimeters below the upper contact where sandstones are mottled pale purple and green.
This discoloration is probably associated with the top unconformity and deposition of overlying reducing sediments (Duke and Fawsett, 1987).
Here the top of the Medina Sandstone is an erosion surface overlain by a thin (3 to 5 centimeter) dark gray, phosphatic sandy dolostone (Densmore Creek Bed) with prolific Hyattadina brachiopod valves. A thin laminated siltstone rests on the bed at the contact with the greenish gray Neahga Shale, which at this locality is about 1.0 meter thick.
At the base of the Hickory Corners Limestone (Reynales) is a thin (3 to 5 cm) pyritic, sandy limestone packed with black phosphatic pebbles and shell fragments (Budd Road phosphatic Bed). It is overlain by about 40 cm of alternating greenish gray shales and thin limestones capped by a 60 cm-thick ledge of nodular crinoidal pack- and wackestone at the top of the road cut. These beds contain a prolific fauna including corals, brachiopods (Hyattidina, Dalejina, Platystrophia) and pentameric crinoid stems belonging to a newly described disparid, Haptocrinus. The upper surface at this locality is a glacially striated pavement and the post-Reynales erosion surface cannot be observed.

STOP 2B. Lockport Junction Roadcut (Upper)
(Upper Rochester, Decew, Gasport Formations)
This large road cut displays the upper part of the Rochester Formation (about 5 meters), DeCew Dolostone (2.5 meters), and Gasport Limestone (over 7 meters); it was described in details as Stop 1 of Brett (1982 NYSGA Guidebook). However since that time the exposure has been freshly blasted to widen Route 93.
A subsequence boundary occurs between the dolomitic calcisiltites and shales of the upper Rochester Shale and the overlying DeCew Dolostone. The latter is a buff-weathering, silty dolostone with thin layers of intraclasts and convoluted bedding. The basal contact is sharp and locally channeled, but nearly conformable. The upper Rochester and DeCew are burrowed in some levels, body fossils are rare but this exposure has produced long columns of crinoids in the lower DeCew.
The sharp and undulatory upper contact of the DeCew Dolostone with the Gasport Formation forms the boundary between the Clinton and Lockport groups, and is interpreted as a sequence bounding unconformity (between sequences V and VI). The surface represents an abrupt lowering of relative sea-level and beveling of older strata. The basal Gasport is a greenish gray brachiopod-rich, crinoidal grainstone conglomerate with dolostone clasts eroded from the DeCew. Missing at this contact is the newly recognized Glenmark Shale, a fossiliferous gray shale with litho and biofacies resembling the Rochester Shale.
Only the lower (Gothic Hill) member of the Gasport is present here. At this locality and at cuts along adjacent Gothic Hill Road this unit is exceptionally thick (7-8 meters) and composed of well to poorly sorted pelmatozoan pack- and grainstones. This facies is interpreted as a high-energy crinoidal bank. Brachiopods are common in an argillaceous, thin-bedded unit near the top of the unit. A small bioherm composed of algal and bryozoan boundstone (micrite) occurs within the Gothic Hill Member. The overlying units will be examined at the next stop.

Junction Thrall Road on right.
Exposures of fossiliferous Rochester Shale along Thrall Road were described as Stop 2 in Brett (1982).

STOP 3. "PEKIN BIOHERM" CUT ON RT. 429
Location: Cuts on both sides of NY 429, just north of underpass beneath Upper Mountain Road, Pekin, Niagara County, NY (Cambria 7.5' Quadrangle).
Description: This classic cut in the brow of Niagara Escarpment has recently been widened, and provides an excellent, but very fresh exposure of the upper Gasport (Pekin Member of Brett et al., 1995) and the lower portion of the overlying Goat Island Formation (Fig, 19). The large, massive mound exposed particularly on the west side of Rt. 429 was originally described as a Gasport bioherm and was considered to show reefal succession (Crowley and Poore, 1974). However, recent excavations have revealed that the mass of stromatoporoid bearing rock is almost entirely within the Goat Island Formation and is separated from older Gasport deposits (including a formerly exposed small bioherm, now destroyed by blasting) by a major erosion surface. The contact between the gray, thin bedded argillaceous dolostones of the Pekin Member of the Gasport and overlying massive, dolomitic crinoidal grainstone of the Goat Island (Niagara Falls Member of Brett et al., 1995), a sequence boundary, appears sharp, but horizontal, in the northeastern portion of the cut. However, this contact abruptly descends to near road level just north of the Upper Mountain Road overpass. Here a mass of biohermal lithology overlies the unconformity. On the freshly blasted west side of the cut the irregular contact between the dark gray Pekin and light pinkish biohermal Goat Island is now very clear. The erosion surface separating the units has a relief of over 2 meters. The Goat Island "bioherm" appears to have developed in low areas on the unconformity. It consists of a mixture of crinoidal grainstones and light gray dolomicrite, rich in stromatoporoids, many of which are tumbled onto their sides or even inverted, indicating storm disturbance. This biohermal mass built up in "channels" along a sequence boundary during initial transgression.

Good view of Niagara Gorge to right

Pass forebay on left; driving over sluiceways of Robert Moses Power Plant

Pass Niagara University on left

Pass by fisherman's parking area
Pass by fisherman's parking area (cars can be left here; we will continue on South Haul Road to the bottom of the gorge and allow passengers to disembark and return uphill on foot); ahead through underpass beneath Robert Moses Parkway are excellent views of the Niagara Gorge and a complete stratigraphic section from the mid Silurian Lockport Group down to the unconformable contact between the Silurian Whirlpool Sandstone and the underlying Ordovician Queenston Shale.

Fishermen's access at bottom of Niagara Gorge
Fishermen's access at bottom of Niagara Gorge, just outside gates for Robert Moses Power Plant; passengers will disembark and proceed back up the South Haul Road; vehicles will return back up the road to park

STOP 4. Niagara Gorge - South Haul Road
Location: Large road cuts in east wall of Niagara Gorge along South Haul access road for Robert Moses Power Plant, and ascending for about 1 km south to a tunnel beneath Robert Moses Parkway (Fig. 21). Parking is available in a fisherman's access parking lot just west of Rt. 62 (Hyde Park Boulevard) immediately south (uphill) from the tunnel. Lewiston, Niagara County, New York (USGS Lewiston 7.5' Quadrangle). Note: Access to the Haul Road exposure is strictly controlled by the Robert Moses Power Project and requires advance permission.
Description: This outstanding outcrop of Lower to mid Silurian strata shows important contrasts with sections of the comparable interval near Hamilton. The section begins near the Power Plant with about 8 m of the Upper Ordovician Queenston Shale (Fig. 21). Its sharp contact with the overlying Whirlpool Sandstone is the Cherokee Unconformity (Silurian Sequence I boundary).
The units of the Silurian succession are described in ascending order, as follows:
Medina Group (Fig. 21).
Whirlpool Sandstone: (4.5 m)- White, trough cross bedded, quartz arenite facies which record a non-marine to marine transition. Excellent profiles of channels are visible.
Power Glen Shale: (∼8 m)- Dark gray, friable shale, with very minor sandstone interbeds
Devils Hole Sandstone: (2m)- Pale gray, massive, quartz arenite with a distinctive, meter-thick phosphatic, sandy dolostone, Artpark Phosphate Bed near the top (IC).
Grimsby Formation: (15 m)- Greenish gray to maroon shales and mudstones with bundles of thin reddish and white mottled sandstones (Fig. 21).
Thorold Sandstone: (2m)- White, cross-bedded quartz arenite. The Thorold has a sharp, erosive base which marks the base of the next Medina subsequence (IC). A thin (2-10 cm), sandy phosphatic bed, the Densmore Creek Bed (Brett et al., 1995) rests sharply on the Thorold (and on a Cambria Shale remnant north of the Power Plant), marking the base of the Neahga Shale. Clinton Group (Fig. 18)
Neahga Shale: (2m)- Dark greenish gray, very friable shale (base sharp (II SB). Reynales Formation (Hickory Corners Member): (∼50 cm)- Medium gray, nodular, burrowed, bryozoan-rich wacke- to packstone. Conodonts indicate a mid Llandovery age for the Reynales (see LoDuca and Brett, 1994); this unit represents an erosional remnant of the Reynales.
Rockway Formation: (3 m)- Buff-weathering, argillaceous dolostone with thin dolomitic shales shows prominent rhythmic bands (10-50 cm) of sparsely fossiliferous argillaceous dolostone interbedded with thin gray shales. The Rockway shows a sharp upper contact (S-IV-V SB).
Irondequoit Formation: (2.5 m)- Massive, pinkish-gray, crinoidal pack-and grainstone. Clasts of fine-grained dolostone, derived from the underlying Rockway occur in the basal thin bed of the Irondequoit (Fig. 23). Its sharp upper contact (MFS) is marked by a 30 cm thick shell bed.
Rochester Shale: (18 m)- Medium dark gray mudstone with thin calcisiltites and lenticular fossil rich limestones; 1.5 m of bryozoan-rich limestone beds underlie the sharp top (MFS) of the Lewiston Member. The upper Rochester (Burleigh Hill Member) also displays a sharp contact (SSB) with the enterolithic DeCew Dolostone beds.
DeCew Formation: (3 m) - Dark gray, buff weathering, laminated dolostone (calcisiltite). Here and, especially in weathered exposures in the adjacent Devils Hole Park section, the DeCew displays spectacular soft- sediment deformation with isoclinally folded beds (seismite?).
Lockport Group (Fig. 24): The sequence V-VI boundary at the DeCew-Gasport contact is well exposed near the entrance to the "tunnel" beneath the Robert Moses Parkway at the top of the cut; basal Gasport shows rip-up clasts of DeCew Dolostone (Fig. 17).
Gasport Formation: (5 m)-Pinkish gray thin bedded to massive dolostone, divided into a lower pinkish gray dolomitic crinoidal grainstone (Gothic Hill Member) and a 2.5 m upper argillaceous, bioturbated dolostone (Pekin Member) Weathered surfaces of the Gothic Hill grainstones display probable bipolar cross- stratification . The sharp upper contact is a subsequence boundary.
Goat Island Formation: (∼10 m) - Buff weathering dolomitic, crinoidal grainstones, buff, thin bedded dolostone with white chert, and dark brownish gray, argillaceous, banded dolostone; the basal unit (Niagara Falls Member) is massive crinoidal grainstone with scattered Cladopora corals and stromatoporoids; abundant vugs appear to be solution cavities in a stromatoporoid-rich zone. The Lancaster Member is poorly developed here, thin (∼2.5 m) and only sparingly cherty. Argillaceous and bituminous gray dolostones of the Vinemount Member form the uppermost unit on the access road. This member is much less shaly than at its type area near Hamilton.

STOP 5. Robert Moses Access Road and Forebay
Location: Access road to the Robert Moses Power Plant, and adjacent forebay canal just west of Military Road and 1.2 km north of Route 31, Lewiston, Niagara Co., NY (Lewiston 7.5' Quadrangle).
Description: Higher units of the Lockport Group are visible along the forebay and consist of the upper Eramosa (new usage in New York) and lower Guelph formations (Fig. 24). Large algal bioherms characterize the uppermost units of the Eramosa Formation in the forebay. Exposures in the small road cut at the underpass of the access road beneath the lanes of I-190 show exceptionally large (2 m high) stromatolites, and some non-laminated thrombolites. The stromatolitic interval at the base of the Guelph is traceable with subsurface data at least to Hamilton (Fig. 24).

Crossing east end of Robert Moses forebay area
Pump Generating Power Plant to right; note small thrombolitic mounds in Eramosa Formation exposed in forebay cuts to left

Brow of Niagara Escarpment
On clear days good views of the mouth of Niagara River and Lake Ontario to the north; cuts to right in Gasport-Goat Island formations

STOP 6. Niagara Gorge, Lewiston (Artpark)
Location: Sections in east wall of Niagara Gorge along old haulage road extending from north end of Niagara Gorge at Niagara Escarpment just south of Artpark, off Fourth Street, southward to the Lewiston-Queenston bridge, Lewiston, Niagara County, NY (Lewiston 7.5' Quadrangle)
Description: The north-facing cuesta (Niagara Escarpment) stands 76 m above the adjacent Lake Ontario plain. Niagara Falls was initiated here about 12,000 BP.
Exposures of the Upper Ordovician Queenston Shale and its unconformable contact with basal Silurian Whirlpool Sandstone are visible along a short path, adjacent to the river, immediately south of the Artpark theater. Outcrops of the Lower Silurian stratigraphic units above the Whirlpool Sandstone are accessible along an old haulage road that leads southward from the Artpark Visitor Center into the gorge. At the entrance to the gorge (edge of Niagara Escarpment) an isolated "butte" of Lower Silurian strata (Power Glen-lower Grimsby; type locality of the Artpark Phosphate bed) between the path and the river, represents a remnant of a promontory in the gorge wall that was breached during excavation for the power plant. A 0.5 m layer of mottled sandstone with prominent ball-and-pillow structures, about 5.5 m above the base of the Grimsby, is well exposed in the cliffs about 200 to 700 m north of the Lewiston-Queenston Bridge. Caution is required in this exposure, as rock falls are common. Abundant fallen debris also provides an excellent look at varied lithologies of the upper Medina, as well as Clinton and Lockport units.