A4: CLASSIC LOCALITIES OF THE BLACK RIVER AND TRENTON GROUPS (UPPER ORDOVICIAN) IN THE BLACK RIVER VALLEY: REVISITED THROUGH TRADITIONAL AND SEQUENCE STRATIGRAPHY
Sean Cornell
Geology Department, St. Lawrence University
Joseph Andrews
Department of Education, LeMoyne College
Paul Agle
Department of Geology, State University of New York at Buffalo
David Thomas
Department of Earth Sciences, State University of New York at Oswego
INTRODUCTION
The late Ordovician (Mohawkian Series) Black River and Trenton groups of the northern New York State consist of some 250 m of highly fossiliferous, well-preserved limestones with some intervening shales. These rocks are exposed along a southeast-northwest trending outcrop belt for more than 250 km in central New York and neighboring Ontario, where they closely flank the Precambrian Grenville Province (Figure 1). These units deposited from ∼457-450 million years ago, have been studied for nearly two centuries and have become very important units for the study of paleontology, carbonate sedimentology, stratigraphy, paleoclimatology, paleoceanography, paleoecology, structural geology, and more recently, petroleum and natural gas geology. Mostly known from the renowned Mohawk Valley sections including those at Trenton Falls on West Canada Creek, the Black River and Trenton play (and have played) an important role in our understanding of depositional processes, environmental change and even mountain building and plate tectonics. The purpose of this field trip is to revisit the classic stratigraphy of the Black River and Trenton groups, and to introduce some of the modern research ideas that are being applied to answer long standing questions regarding the history of these rocks.

A4: CLASSIC LOCALITIES OF THE BLACK RIVER AND TRENTON GROUPS (UPPER ORDOVICIAN) IN THE BLACK RIVER VALLEY: REVISITED THROUGH TRADITIONAL AND SEQUENCE STRATIGRAPHY

STOP 1. Lowville, NY, Mill Creek Gorge
This locality represents the type locality for the Lowville Formation and although the original quarry section is flooded (it is just across the road from the gorge) the gorge offers excellent exposures and weathered outcrops for the review of sedimentary structures and fossiliferous strata. The section starts in the Pamelia Formation and intermittently in the banks at the lower end of the gorge. It is possible to observe the uppermost Pamelia Formation. Exposures of the “Depauville” Member and overlying Pittsburgh Quarry Bed of Conkin and Conkin (1991) can be identified.
Up in the narrows of the gorge, exposures of the lower Lowville Member or “Birdseye” of early workers can be seen in a series of well-developed meter-scale cycles, each with a flooding surface at their base and a shallowing upward motif. The Upper Lowville, or House Creek Member (of Walker, 1973), is fairly well exposed in the southern wall of the gorge and although it can be difficult to access, this member of the Lowville is distinctively less argillaceous and more massively bedded. It is composed almost exclusively of bioturbated wackestones with minor stringers of intraclastic packstones, and a few ribbon bedded horizons showing some ooid development. Overall the House Creek represents protected, but marine conditions typical of shallow water lagoon areas. Although it is much more subtle and is occasionally recognized as one more cycle of the Lowville, the uppermost beds of the House Creek, or the “Weaver Road Beds” show the distinctive development of domal stromatolites in fairly argillaceous platy micrites. Overlying the Lowville Formation is a series of more massive-bedded limestones previously referred to the Watertown Limestone. Although much of this particular interval is extremely cherty, the basal few feet are less so and are fairly coarse-grained. In stark contrast to the underlying lithologies, the basal few feet of the upper massive unit are characterized by large colonies of Tetradium, crinoids, brachiopods, a variety of high-spired gastropods and numerous other taxa. Given the faunal and lithologic composition of this bed and through correlations with sections to the northwest in the Watertown area, it is believed that this thin interval of beds represents what is left of the Leray-Glenburnie interval. Without the occurrence of the Hounsfield K-bentonite (sensu Kay, 1931), in this succession it is surmised that the Glenburnie and the K-bentonite have been eroded by the overlying sequence boundary and were removed prior to the deposition of the Watertown. The Watertown is the last well-displayed unit is this portion of Mill Creek. Just below the Rt. 12-26 bridge over Mill Creek, the south wall of the gorge displays the massively bedded and chert-rich equivalent of the Watertown Limestone. Approximately 4 m of Watertown are exposed and are typically well bioturbated wackestone to packstone facies. Like the House Creek, the Watertown contains many coral taxa, sponges and stromatoporoids, and more diverse brachiopod and crinoid assemblages. Based both on lithologic and faunal patterns, the Watertown itself demonstrates a significant and even more substantial upward deepening pattern. Although many of the internal cycles have become amalgamated, they can often be identified through the recognition of firmground and hardground surfaces. These surfaces become especially prominent near the top of the succession where they show evidence for phosphate, pyrite, glauconitic staining, and may even contain K-bentonites sitting on them. Moreover, these surfaces often are accentuated by hard-substrate borings and in some instances have been found to contain crinoid & bryozoan holdfasts indicating that these surfaces were well cemented on the sea-floor prior to their final burial. Collectively then, the Watertown itself is clearly an upward deepening unit and although it does not represent the deepest portion of the sequence, it is the deepest facies within the entire Black River to this point.
Near the top of the gorge section below the bridge, a small poorly-exposed succession of rubbly weathering wackestone-packstone lithology stands in stark contrast to the underlying massive bioturbated beds of the Watertown. This particular interval becomes very condensed in places and shows evidence for sediment starvation through the accumulation of numerous current aligned cephalopods (with a variety of taphonomic signatures), and a range of diminutive orthid, and strophomenid brachiopods (Dalmanella, Sowerbyella respectively). This unit is referred to as the Selby, and is included in the Rocklandian Stage. Lithologically and faunally, the Selby is transitional out of the Watertown into the overlying Napanee Formation. Unfortunately, poor exposures above the bridge do not permit the observation of the upper Selby to Napanee interval.

Parking for stops 2a and 2b

STOP 2a: East Martinsburg, NY
ROARING BROOK, LOWER SECTION Just east of Route 12 on the lower section of Roaring Brook is the unconformable contact of the Pamelia Formation with the underlying Precambrian Grenville Gneiss. Although exposure here is covered for some distance above the contact, a few feet of the Pamelia are exposed and document the transition from a siliciclastic sandy (dominated by quartz and feldspar fragments) carbonate upward into more medium-bedded argillaceous limestones. Although very few fossils are found in this unit, occasionally a few gastropods, ostracods, and small fragments of bryozoans are represented. These are commonly of poor preservational style and many are coated with greenish glauconitic material. Although the depositional history of this lowermost Pamelia is not well constrained, it is very similar to strata exposed in southeastern Ontario where this particular interval is located on the top of the Shadow Lake and is time equivalent with the basal Gull River Formation. Here in New York, the basal Pamelia sandy limestones are thought to represent the deepest phase of the Shadow Lake or Rideau sequence and overall remain fairly shallow but mudcracks a few feet off the basal sands suggest that this may be a shallowing succession off the basal transgressive surface. That is the red and green shaly carbonates of the Shadow Lake (which are not developed here in the central Black River Valley) were probably deposited during the transgressive phase, while the overlying lower Pamelia was deposited during the maximum inundation of that particular sea level rise event.

STOP 2b. East Martinsburg, NY, Roaring Brook
MIDDLE SECTION Up section, the remainder of the Black River Group is exposed through the top of the Watertown Limestone. The middle-upper division of the Pamelia is well represented and overall represents one transgressive-regressive sequence. Although not formally named, the middle or “Thompson Farm” member is a distinctive facies of the Pamelia and is well developed through the transition from laminated argillaceous micrites and dolomicrites to more medium-bedded (and distinctively less argillaceous) wackestones containing a fairly restricted fauna but occasionally contains a few echinoderm fossils (crinoids), a few large strophomenid brachiopods, very large endoceratid cephalopods, and several trilobite taxa. In contrast to the more restricted ostracod-bivalve-gastropod assemblages of both the lower and upper Pamelia, the middle member is characteristically more normal marine in faunal composition. Although it is still considered a fairly shallow peritidal deposit, the presence of these faunal elements indicates that the middle Pamelia was deposited in deeper water. A fairly rapid and substantial transition upward into the upper Pamelia or “Depauville Waterlime” of early workers is documented by the transition back into dolomitic limestones and sandy dolomites characteristic of this interval. In addition, distinctive sedimentary features suggest that this particular interval shows evidence of hypersaline conditions. In some beds evaporite crystal pseudomorphs are distinguished as are the development of large vugs containing celestite and/or gypsum. Although it is clear that many of these have been recrystallized during later diagenesis, the moldic preservation of some of these crystal faces indicates that these environs may have been highly saline at the time of deposition.
Above the Pamelia, both the lower and upper Lowville members are well exposed and with some investigation are readily compared to sections visited at Mill Creek in Lowville. Unfortunately, due to extreme solution along joints in the stream bed, many of the sedimentary and faunal details of the Lowville are obscured but many are still readily observable. In this region, the Lowville is approximately 25 m thick. Near the top of the succession, an argillaceous, mud-cracked interval at the base of the Watertown is recognized. Although there are no large domal stromatolites (as in the northern Black River Valley), there is evidence for low, broad stromatolitic mounds (LLH style) in this interval. These stromatolites border on thrombolitic textures where they are burrowed and are evidently grazed.
Continuing upstream are exposures of the Watertown Limestone. Again as below, some local small-scale structures are developed in these exposures. Although most appear to be related to differential compaction over shallow Precambrian basement features, it is also evident in this section that there are minor faults that offset strata locally. It is not clear at this time if these are related to more recent uplift associated with Neogene to recent Adirondack tilting or if these are representative of local faulting at or near the time of deposition in the Upper Ordovician. Nonetheless, the Watertown and overlying Selby Formation represent a continuous deepening showing the upward transition from typical bioturbated cherty wackestones and packstones of the upper Watertown into more nodular, rubbly weathering Selby. The Watertown itself is substantially thinner than the Watertown in the type region and it is believed that only the upper portion of the Watertown is here by overlap. At the top of the section, as is the case in most of the region, the top Selby forms a broad terrace where the overlying shaly bedded Napanee Formation is eroded back almost a half mile. Although we will not trek overland to the next exposure, the basal beds of the Napanee are intermittently exposed just upstream.

STOP 3. Whitaker Falls Park, Martinsburg, NY
ROARING BROOK UPPER SECTION Upstream from the previous stop, we start just above the lower waterfall in Whitaker Falls. Although we could travel downstream, for the sake of time, we will focus on the interval from the lower falls upward through the upper Trenton. This portion of Roaring Brook exposes the entirety of the Shermanian from the Sugar River at the base through the Denley, Rust, and finally the Steuben formations. We will not see the Kings Falls or Napanee up close at this locality although they are exposed downstream (we will see them at the STOP 4 in Boonville, NY). Exposed in Whitaker Falls Park are three closely spaced waterfalls in this portion of the gorge. Just below the lip of the first waterfall, and exposed in the south wall of the gorge are the uppermost beds of the Kings Falls (forming the floor below the falls) and the Lower Sugar River formation. The shaly nature of the Lower Sugar River allows for it to be slightly less resistant and allows for the notch under the waterfall. Also exposed in the wall of the gorge at this point are at least two recessive weathering notches and a deformed channel. The channel and the layer above the waterfall show evidence for brecciated fabrics suggesting syn-sedimentary movement of materials on the sea-floor after some period of lithification. Although it is not yet clear as to the regional relationship of this feature, both of these features are found in many local sections throughout the region and are prominent horizons useful for correlation.
The lowermost waterfall is capped by the Rathbun Member of the Sugar River Formation and near the top of the middle waterfall is the transition to the upper Poland. Just above the lip of the middle falls and below the bench leading to the upper waterfall is the transition to the Upper Denley (Russia Member). Within the face of the third falls are the well developed cycles of the Russia and the basal Mill Dam Member of the Rust Formation. The fairly coarse cycle caps of the Mill Dam help to hold up the face of the falls and are exposed for some distance upstream where excellent large-scale ripples are well developed on them. Like the coarse-grained Kings Falls some distance downstream, the Mill Dam in this region was substantially shallower than the underlying Denley Formation, although it too shows an upward deepening pattern.
Continuing upstream some distance exposures in the wall of the gorge afford views of the Rust Formation and its members. In the walls of the gorge before the Glendale Road Bridge, exposures of the Mill Dam and Spillway members show characteristic coarser-grained facies with well preserved faunas, extremely large brachiopods and some crinoids. Another deformed interval is located here and is tentatively correlated with the uppermost Spillway member slump structure at Trenton Falls. Just above the bridge, exposures of the Prospect Quarry member give way to the basal coarse-grained Steuben Formation. Although an additional 25-30 meters of section are exposed upstream near the Martinsburg town garage (as is the contact with the overlying black shales), owing to time constraints we will defer their investigation at this time.

STOP 4. Boonville NY
BARRETT MATERIALS AND PAVING QUARRY AND SUGAR RIVER SECTIONS
Just north of the village of Boonville, NY on the north shore of Sugar River sections are exposed in the Barrett Materials and Paving Quarry and in the adjacent stream section to the south of the quarry and along route 12. Entrance is by permission only and hard hats and steel-toed shoes must be worn at all times. Although exposures in this section extend down to the base of the Pamelia, we will focus on the uppermost Black River Group exposed in the quarry. In the older northwest corner of the pit and along the west wall of the pit, exposures start in the lower Lowville and extend upward through an abbreviated House Creek section where the Watertown Limestone, or “7’ tier” of quarry operators, immediately overlies the middle House Creek with possible unconformity. Although the 7’ tier is dominantly a single massive bed composed of fairly coarse micritic wackestone, in the face of the unit to the southwestern corner of the pit, the upper massive bed displays a prominent recessive notch containing a prominent clay horizon. Although it still remains to be chemically fingerprinted it sits in the same position as a K-bentonite approximately 1.5 meters below the top of the Watertown at Brownville gorge. In the older portions of the quarry, the K-bentonite has been squeezed out and it is difficult to make out, as is the case in most of the weathered stream sections.
Although no higher exposures are afforded in the quarry–the cap of the quarry wall is the Watertown– additional exposures in the stream bed allow for the continuation of the stratigraphic section. Just to the south of the quarry in the stream bed and southern bank of the Sugar River, the upper contact of the Watertown with the Selby is observed. The massive Watertown is easily recognized in the bed of the stream and along the southern bank a several decimeters of thin-bedded rubbly weathering limestone is exposed below the typical shaly-interbedded Napanee Formation. At this particular location, only the basal 2-3m of the Napanee is exposed in the outer bend of the stream. Upstream, above the NY 12 bridge, additional sections are exposed from the Napanee up through the King Falls Formation. W will not walk upstream to see them in weathered section, but we will look at the same interval in the road cuts on both the north and southwest side of the highway. Here in the road cuts are the uppermost ∼5 meters of Napanee followed by ∼3 meters of Kings Falls. Although not in the best position for weathering, the outcrop shows the sharp contact of the Kings Falls on the underlying Napanee. In fact, the contact shows evidence for channeling into the underlying Napanee at the M5A-M5B sequence boundary. Additional exposures are visible upstream, including the type succession of the Sugar River Formation, and additional units up through the upper Trenton.