10: REFINING THE TIMING OF FAUNAL TURNOVER IN THE MIDDLE DEVONIAN APPALACHIAN BASIN: PALEOECOLOGICAL ANALYSIS OF THE EARLIEST HAMILTON FAUNA AND A REVISION OF THE BASE OF THE GIVETIAN STAGE IN EASTERN NORTH AMERICA
Alex Bartholomew
Thomas Schramm
State University of New York at New Paltz
Charles Ver Straeten
New York State Museum
Jeffery Over
State University of New York at Geneseo
INTRODUCTION
The Middle Devonian of the Appalachian Basin has long been a testing ground for various evolutionary and paleoecological hypotheses (Eldredge and Gould, 1972; Brett and Baird, 1995; etc.). Units such as the Onondaga Limestone and Hamilton Group contain an excellently preserved invertebrate fauna spanning a wide array of marine facies ranging from shallow, reefal environments with diverse faunas to deep-water, dysoxic settings typified by nearly monospecific assemblages. The well-documented nature of the stratigraphy of the Middle Devonian rocks of New York State in particular makes this interval the perfect area in which to investigate the spatial and temporal nature of paleoecology and faunal turnover during this critical interval in Earth history.
Workers such as Cleland (1903) recognized early on the presence of patterns of extended intervals of faunal stability within the Middle Devonian of New York State, patterns that lead Brett and Baird (1995) to the formation of the hypothesis of Coordinated Stasis. Their hypothesis states that there are extended intervals of compositional faunal stability, termed evolutionary-ecological subunits (EE subunits), within various environments throughout geologic time, that are bounded by relatively rapid pulses of faunal turnover with very low amounts of taxonomic hold-over between stable intervals.
More recent investigations on the topic of Coordinate Stasis have focused on gaining a better understanding of the type interval from which Brett and Baird formed their hypothesis within the Middle Devonian of the Appalachian Basin. Bonnelli et al. (2006) documented the nature of abundance patterns within the Hamilton EE subunit, noting that faunas from similar paleoecological settings had high levels of compositional stability but low levels of stability when taking abundance of the taxa into consideration; although the composition of the top ten taxa changed little from bed to bed, the abundance ordering between beds was quite different. A more recent study by Brett et al. (2007) focused on patterns of biofacies tracking within the Hamilton EE subunit, noting high levels of similarity of taxonomic relative abundance within biofaces through depositional cycles. While these studies have documented the nature of the intervals of faunal stability, the intervening periods of faunal turnover remain as yet poorly understood. The present study aims to shed light on the details of the faunal turnover between two of the EE subunits with the Middle Devonian of the Appalachian Basin, the Stony Hollow and Hamilton EE subunits, specifically examining how rapid was that turnover and how quickly were the previously documented suit of biofaces from the Hamilton interval established within the basin.

10: REFINING THE TIMING OF FAUNAL TURNOVER IN THE MIDDLE DEVONIAN APPALACHIAN BASIN: PALEOECOLOGICAL ANALYSIS OF THE EARLIEST HAMILTON FAUNA AND A REVISION OF THE BASE OF THE GIVETIAN STAGE IN EASTERN NORTH AMERICA

Upper Ordovician Martinsburg Shale outcrops

Cross Rondout Creek

Lower Devonian Port Ewen Fm. outcrops

Lower Devonian Port Ewen, Alsen, and Becraft Fm.'s

Lower Devonian Schoharie Fm.

Lower-Middle Devonian Onondaga Fm.

Onondaga Fm

Cross Esopus Creek

STOP 1. City View Terrace.
Our first stop this morning will be used to set up the stratigraphic and faunal story that will be the focus of today's trip. The outcrop along City View Terrace consists of shales and siltstones of the middle Union Springs Fm. and is the type locality for the base of the Stony Hollow Member with the upper-most portion of the underlying Bakover Member also being exposed (Figure 5). The contact between the two formations is marked by a layer of carbonate concretions as well as a discrete color change from dark-gray to black shales below to lighter-gray shales above; the concretions contain rare goniatites (Cabrieroceras) (Figure 18). This outcrop is a good example of the sediments that were deposited during the initial phases of the second tectophase of the Acadian Orogeny (Ettensohn, 1985). The downwarping of the edge of the craton due to back thrusting of the magmatic arc to the east created an over-deepened basin containing dysoxic to anoxic bottom waters allowing little to no benthic faunas to exist. Associated with this initial tectonic deepening is an eustatic sea level rise that brought warm, equatorial waters and associated faunas down to the south into the Appalachian Basin during this time; this is the local manifestation of the Kacak Bioevent. We see the first influx of warm-water adapted taxa in the upper Union Springs with the appearance of pelagic cephalopods previously unknown in much of the basin, such as Cabrieroceras. Additional taxa invade into the basin, displacing the preexisting cold-water adapted taxa, with a moderately diverse benthic fauna being present in the overlying Stony Hollow Member; this fauna has been termed the Stony Hollow Fauna (Figure 2). Today we will be primarily examining the interval that records a return to more 'normal' conditions for the Appalachian Basin near the end of the Kacak Bioevent showing a return of cold-water adapted taxa into the basin (Figure 4) associated with basin shallowing and associated sea level fall within the overlying East Berne Member of the Mt. Marion Fm.

Middle Devonian Stony Hollow Member of Union Springs Fm.

Middle Devonian Otsego Member of Mt. Marion Fm.

STOP 2. Route 209.
Halihan Hill Bed in outcrop to right marking boundary between the East Berne Member (below) and Otsego Member (above) of Mt. Marion Fm.
At this stop we will be dropped off at the southern end of the outcrop and walk upwards through the section from the upper Stony Hollow Member, through boundary of the Union Springs and Mt. Marion Fm. at the base of the Cherry Valley Member, then into the overlying East Berne Member with a fine exposure of the Dave Elliot Bed, finishing up at the Halihan Hill Bed at the base of the Otsego Member where we will re-board the vehicles (Figure 5). Starting at the southern end of the outcrop we are in the upper siltstones and sandstones of the Stony Hollow Member (Figure 19). The unit is bioturbated and contains several shelly horizons including the lower and upper Proëtid beds containing a diverse example of the Stony Hollow Fauna (Figure 2). In the interval between the upper Proëtid Bed and the base of the Cherry Valley Member is the Hurley Member of Ver Straeten et al. (1994). This interval contains the distinctive goniatite Agoniatites nodiferans. Only the lower ∼2 m of the Cherry Valley Member is exposed here consisting of thick-bedded sandstone with a depauperate fauna of dominantly leiorhynchid brachiopods (Cherryvalleyrostrum).
Sadly, the contact between the Cherry Valley Member and the overlying East Berne Member is covered here, but only for ∼10 m or so. The lowest portions of the East Berne Member are medium-gray, fine-grained, very fissile shales that grade upwards with more and more silts. Near the middle of the member, we see the entrance of thicker-bedded siltstones and fine-grained sandstones. These beds are very homogenous inside and show little to no burrowing in nearly every instance at this outcrop. We interpret these beds to be storm-derived sediments that were most likely deposited nearly instantaneously into deep, dysoxic waters. Continuing up section we first come to a distinct rusty horizon that contains an incredible abundance of infaunal nuculid bivalves (Nuculoidea) (Figure 20). Approximately 2 m above this bed is a distinctive horizon of vertically-oriented concretions formed around tubular pyritic burrows. This bed is ∼1 m below the Dave Elliot Bed, another rusty horizon with abundant nuculid bivalves and rare leiorhynchid brachiopods and cephalopods. This bed represents a 'dirtier' water version of the leiorhynchid biofacies of Brett et al. (2007) (Figure 15). Continuing upwards we see thicker and thicker siltstone and sandstone beds until near the northern end of the outcrop we encounter the diverse Halihan Hill Bed that contains abundant spiriferid brachiopods and common small rugosan corals.

Stony Hollow Member

Cross Sawkill Creek

East Berne Member

Cross Plattekill Creek

Otsego Member

Mt. Marion to west of Thruway

Big Quarry in East Berne Member on north end of Mt. Marion

REST STOP at Stewart's/MacDonald's

Onondaga Fm.

Kaatsban Stone Church (Kaatsban is Dutch for tennis court)

STOP 3. Dave Elliot Quarry.
Stop 3 is the type locality of the Dave Elliot Bed along Dave Elliot Road (Figure 5, 21). This small quarry exposes ∼10 m of the upper East Berne Member and dip markedly to the west. The Dave Elliot Bed consisting of a ∼20 cm siltstone bed containing an abundance of chonetid brachiopods exposed near the eastern end of the quarry, holding up a small terrace; many of the brachiopods in the bed are distinctly white in color at this locality. Also present in the fauna of the bed here are rare to common spirifers and common emmanuellid brachiopods. The biofacies preserve here would be consistent with the chonetid biofaces of Brett et al. (2007) (Figure 15).

Cross Kaaterskill Creek just above High Falls

Stony Hollow and Cherry Valley members
Base of Cherry Valley Member marks the base of the Mt. Marion Fm.

Upper Proetid Bed of Stony Hollow Member

STOP 4. Sandy Plains Road.
The Dave Elliot Bed at Sandy Plains Road (Figure 5) is exposed at the top of a ∼1.5 m sandstone bed and is dominated by crinoidal debris, abundant spiriferid brachiopods along with with rare bryozoans. It is interesting to note that the upper portion of the East Berne Member has coarsened considerable in this area, with the sandstone containing the Dave Elliot Bed thickening by almost an order of magnitude (Figure 22). The biofacies preserved in the bed here most closely resemble a moderately diverse brachiopod assemblage of the Hamilton Group (Figure 15).

LUNCH STOP - Butter Milk Falls.
Our lunch stop today is a picturesque spot at the Buttermilk Falls preserve just north of Leeds (Figure 5, 23). Exposed in the stream is the uppermost portion of the Stony Hollow Member and the overlying Cherry Valley Member. Continuing upstream the East Berne is exposed on private property and consists of silty shales with numerous thin-bedded sandstones. The main face of the falls where we will eat lunch is the upper Proëtid Bed and the distinctive brachiopods Variatrypa arctica and Pentamerella cf. wintereri have been found here representing the Stony Hollow Fauna. The smaller falls just upstream from the main falls is the Cherry Valley sandstone and is distinctly cleaved at this locality. The Dave Elliot Bed is present upstream and forms another ∼4 m waterfalls and very closely resembles that seen at Sandy Plains Road just to the south (Figure 24).

Onondaga Fm. outcrops all along this road

Quarry on left, Albany Co. line

STOP 5. Gedney Hill Road.
Stop 5 today is an exposure of the middle portion of the East Berne Member along Gedney Hill Road (Figure 5). We have skipped what are perhaps some of the most interesting exposures of the Dave Elliot Bed in the intervening area between Buttermilk Falls and here, however access to these exposures is limited at this time. We will walk up through the entirety of the exposure of the East Berne Member here as parking is limited. The Dave Elliot Bed is here represented by a ∼1 m very hard quartz-rich sandstone bed (Figure 25) and contains an abundant fauna of brachiopods (Athyris, Mediospirifer, Mucrospirifer, Tropidoleptus) and rare corals and conulariids and would sit somewhere between the diverse brachiopod biofacies and the coral bed biofacies in the parlance of Brett et al. (2007) (Figure 15). In between here at Butter Milk Falls are the Earlton and Stanton Hill Ravine sections. The Dave Elliot Bed at Earlton contains the most diverse fauna yet discovered including rare tabulate corals, moderately-common rugosan corals, and abundant spiriferid and athyrid brachiopods. This biofacies is the closest we get within the interval to the coral bed biofacies of Brett et al. (2007) (Figure 15). The outcrop of the Dave Elliot Bed at Stanton Hill Ravine is similar in lithology to the exposure at Gedney Hill Road, however the fauna is less diverse (entirely lacking in corals) and almost entirely dominated by Tropidoleptus. This association would sit on the 'dirtier' side somewhere between the diverse brachiopod and Athyris-Mediospirifer biofacies of Brett et al. (2007) (Figure 15).

End of Gulch Cave to right

Onesquethaw Cave to left

Cross Onesquethaw Creek;
Nice fold in creek in upper Onondaga Fm.

Clarksville Cave

Cross Onesquethaw Creek

REST STOP

STOP 6. Hannacroix Ravine.
The last stop today will be at Hannacroix Ravine (Figure 5). We consider this locality is somewhat of a 'Rosetta Stone' of sorts for the East Berne Member as it contains numerous individual fossiliferous horizons in addition to a number of well-developed small-scale cycles throughout the member. The abundance of thin, shelly horizons within this exposure confused us at first as to the proper identification of the true Dave Elliot interval, however this led to the discovery of some very important fossils that might have otherwise gone unnoticed. The actual Dave Elliot Bed consists of a ∼0.75 m thick silty sandstone with Zoophycus burrows and a fauna dominated by spiriferid brachiopods and conulariids and is underlain by the distinctive vertically-oriented concretions; the bed itself caps a small falls within the creek (Figure 25).
We have, to date, found nine distinct fossiliferous horizons within the middle to upper East Berne Member at this locality (Figure 27, 28). Besides the Dave Elliot Bed, the horizon with the most well-developed fauna forms a ∼30 cm interval with dispersed abundant nuculid bivalves, rare leiorhynchid brachiopods, and common nautiloid and goniatite cephalopods. This interval is underlain by two distinct 'gummy' clay horizons (possible bentonites, see discussion in main body of text). We have provisionally named this interval the Hannacroix Ravine Bed. Of great importance was the discovery at this site of the zonally important goniatite Tornoceras aff. mesopleuron (Figure 29, graciously identified for us by Dr. R. Thomas Becker of Muenster, Germany). At the standard section for the base of the Givetian in Morocco, this taxa first appears just above the base of the P. hemiansatus conodont zone that marks the base of the Givetian Stage and the occurrence of this taxa in the Dave Elliot Bed here marks its lowest record in eastern North America to date and helps to establish the base of the Givetian Stage as occurring somewhere below this bed and above the top of the Cherry Valley Member which contains conodonts indicative of the upper Eifelian (T. kockelianus Zone) and puts the lower East Berne in the P. ensensis Zone (Figure 30).
Between the Dave Elliot Bed and the Halihan Hill Bed at the top of the East Berne, there are at least four distinct fossiliferous horizons, each becoming progressively more diverse as you proceed up-section. Between the Hannacroix Ravine Bed and the Dave Elliot Bed are two, thin fossiliferous horizons dominated by scattered nuculid bivalves. There are also two nuclulid-dominated horizons below the Hannacroix Ravine Bed that can be traced through the ravine and out onto the road where the lowest forms a ∼2 m falls in a side gully entering from a ditch to the south of the main ravine; the upper of the two beds is visible further up the ditch to the south. A more detailed investigation of the entire East Berne Member is planned for this section. To date no fossiliferous horizons have been found downstream of the road culvert, although a picturesque waterfall is present about 100 yards downstream providing an accessible lower section that needs more work.