Composition Along Two Middle Devonian Paleoenvironmentalgradients

224 224 RESEARCHREPORTS REPORTS~~~~~~~~~~~ Comparative Spatial Variability in Faunal Composition Along Two Middle Devonian PaleoenvironmentalGradients APRIL G. LAFFERTY and ARNOLD I. MILLER Department of Geology (ML 13), University of Cincinnati, Cincinnati, OH 45221 CARLTON E. BRETT Department of Geological Sciences, University of Rochester, Rochester, NY 14627 PALAIOS, 1994, V. 9, p. 224-236 INTRODUCTION Previous studies of spatial patterns among fossil assem- Analyses of spatial faunal variability along paleoenvi- blages have demonstrated the utility of gradient analysis ronmental gradients (gradient analyses) permit the cali- as an investigative tool in paleobiology. However, because bration of biological and environmental variability at dis- lateral samples in these studies have routinely been wide- crete times in the geologic past (see Cisne and Rabe, 1978; ly spaced (commonly tens of kilometers apart), there is Rabe and Cisne, 1980; Cisne et al., 1982; Springer and little sense of the degree to which faunas vary locally, Bambach, 1985; Miller, 1988; Miller et al., 1992). To con- relative to variation observed at larger scales along hy- duct a gradient analysis, samples should be constrained pothesized transects. To address this concern, the present temporally and collection should be methodical, when ap- study assesses preserved macrofaunal spatial variability propriate and feasible, across the study area. Obviously, in composition among samples collected at both outcrop studies of spatial variability that utilize samples collected (small) and basinal (large) scales, along two onshore-off- across several horizons risk the possibility of temporal shore transects in the Middle Devonian Hamilton Group overprint on apparent spatial patterns. within the Appalachian Basin. The horizons sampled are A series of gradient analytic studies conducted by Cisne, stratigraphically discrete and temporally constrained. Rabe, and others (e.g., Cisne and Rabe, 1978; Cisne et al., Quantitative analyses of faunal composition among 1982; Rabe and Cisne, 1980) involved the collection of bulk samples demonstrate that: 1) Transitions in composition, samples at several temporally constrained horizons. Tran- associated with the onshore-offshore transect, are readily sects through the Middle Ordovician Trenton Group of discernible at the basinal scale in both horizons. Along eastern New York State were delineated and samples col- the Browns Creek bed transect, faunal associations vary lected at stratigraphic intervals above and below K-ben- from an onshore rugose coral-dominated assemblage in a tonite horizons, as well as along these temporally con- wackestone to an offshore ambocoeliid assemblage in a strained intervals. However, these studies did not employ calcareous mudstone. In the Hills Gulch bed, the faunas lateral sampling at the outcrop scale, and, in most in- range from a rugose coral-meristellid assemblage in a stances, lateral samples were spaced kilometers apart. As crinoidal packstone/wackestone to a diverse brachiopod/ a consequence, there is little sense of the degree to which infaunal bivalve assemblage in a calcareous mudstone. 2) the fauna may have varied locally, relative to variation In most instances, compositional variation from locality observed at larger scales along the gradient. Assessment to locality exceeds the degree of variation recognized of variability at a more limited, within-outcrop scale, may among samples at individual outcrops. However, there be useful for two purposes: 1) To ensure that apparent are exceptions, suggesting that it is important to collect transitions from outcrop to outcrop are generally outside multiple samples from each locality. 3) Possible non-ran- the bounds of "background" variability measured from dom variability at the outcrop-scale is apparent among sample to sample at individual outcrops; 2) To recognize samples at two of the localities (one in each bed), sug- potentially meaningful spatial variability that may be ex- gesting that outcrop-scale variability is worth exploring pressed on a relatively confined scale, perhaps not asso- in more detail in future analyses. ciated directly with the larger-scale gradient. In this study, preserved macrofaunal variability was as- Copyright? 1994, SEPM(Society for SedimentaryGeology) 0883-1351/94/0009-0224/$3.00 SPATIALVARIABILITY IN MIDDLEDEVONIAN FAUNAS 225 CayugaLake Eastof CayugaLake sessed at both the outcrop and basinal scales along two Westof CayugaLake (exceptFir Tree Anticline) andFir Tree Anticline Middle Devonian, onshore-offshore transects in the Ap- , , MentethLimestone Mbr.\ Basin. The horizons the Hills Gulch Run Shale Mbr.Portland Point" palachian sampled, 1 o1Deep lmestone and the Browns Creek beds of the Ludlowville |0 ) Formation, ? TichenorLimestone Mbr. /. are stratigraphically discrete and temporally constrained j,ww , ,,-, ,s r , /ss/ /// ./.. ...... ..f... (Fig. 1). These two beds differ somewhat in inferred depositional history and were selected to determine whether Jaycox Shale Mbr. their contrasting modes of deposition would impart dif- 4'.:::. ...' -...F..... ferent faunal signatures. Using samples collected from these beds, several interrelated patterns were investigated: 1) SpaffordShale Mbr. SpaffordShale Mbr. The spatial scales at which paleoenvironmental gradients "barren" \ fossiliferous" are identifiable based on faunal composition; 2) The vari- z LimerickRoad bed ? LimerickRoad bed ? ations in environment that produce different spatial faunal 0 transitions at various scales (i.e., within-outcrop versus Romulusblack shale lens A " basinal scale); 3) Similarities and differences among spatial BloomerCreek bed BloomerCreek bed patterns exhibited by the faunas of two separate beds. 0 LL upper siltstone Below, we first discuss the inferred depositional histories ul -j Wanakah Shale Mbr. Point and of the two beds and the meth- -j Ivy lithologies sampled ------------------------------------ Siltstone Mbr. BarnumCreek bed odologies for collecting and analyzing data. Then, the an- ---- ' -- alytical results are presented. They reveal systematic spa- 0 /^~ --~--------------- A. - -j tial variations in macrofaunal assemblages on the basinal 0 EnsenoreRavine bed scale associated with paleoenvironmental gradients in the I greysilty facies lowersiltstone Appalachian Basin. Compositional transitions from local- Mt.Vernon bed j ElmwoodPoint bed Y ity to locality generally transcend the level of faunal vari- blackfacies Ledyard hale Mbr. greyfacies Otisco Shale Mbr. ability recognized at single outcrops, even when localities i greyfacies <s f are closely-spaced. However, the degree of compositional + C~~~~~~~~Chenango Mbr. variability exhibited at the outcrop scale suggests that, to CenterfieldMbr. tcalcareousmudtone) (silttoneine sandstone) portray faithfully faunal transitions along the paleoenvi- .... .. .(silt.ton..-fe. .. *. ..... ...e nt;.'n.Is . V. ronmental gradient, it is essential to collect multiple sam- v v at each At the scale, where local ples locality. outcrop column of the Ludlowville For- microenvironments be to ran- FIGURE 1 -Generalized stratigraphic might expected vary only mation (modified from Mayer et al., in press). Study horizons are noted domly, spatial variability generally appears to be random, with gray shading. except for two localities (one in each bed) that exhibit possible, though admittedly questionable, systematic transitions in faunal composition. careous gray to black shales, concretionary limestones, and GEOLOGIC SETTING thin, continuous limestones (Brett and Baird, 1985; Sa- varese et al., 1986). In western New York, the Ludlowville The rocks of the Middle Devonian (Givetian) Hamilton (Fig. 1) is bounded at the base by the Centerfield Lime- Group of western and central New York State are ideal stone Member and at the top by the Jaycox Shale Member for this study because they permit a high degree of strati- (Baird, 1979; Mayer et al., in press). Further east, the graphic/temporal constraint: several event beds have been formation becomes more heavily dominated by terrigenous identified and correlated across the outcrop belt (Brett et clastics, including siltstones and fine sands (see Clarke, al., 1986). 1903; Kramers, 1970; Baird, 1979; Baird and Brett, 1981; During middle Givetian time, fine terrigenous sediments Kloc, 1983; Gray, 1984). and biogenic carbonate were deposited in the shallow, sub- The two laterally continuous beds in the Ludlowville tropical waters of the northern Appalachian Foreland Ba- selected for study, the Hills Gulch and the Browns Creek sin (Baird and Brett, 1981; Brett and Baird, 1985; Mayer beds (Fig. 1), differ in inferred depositional history. The et al., in press); the study area was a broad, shallow, sed- Browns Creek bed is one of several argillaceous limestones iment-starved shelf bounded on the west and northwest in the lower third of the Centerfield Member. It has been by the craton and on the east by a northeast-southwest interpreted as preserving a single event, probably a storm trending area of active subsidence and deeper water (Baird (Brett et al., 1986; Speyer and Brett, 1986). The lithology and Brett, 1981). Further to the east, the Acadian Orogenic of this bed varies from a crinoid-dominated wackestone at Belt shed sediments into the foreland resulting in a large, western localities to a calcareous mudstone with localized eastward-thickening clastic wedge (Cooper, 1957; Rickard, concentrations of fossil material at eastern localities.

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