Comparative Spatial Variability in Faunal Composition Along Two Middle Devonian Paleoenvironmental Gradients

224 RESEARCH REPORTS Comparative Spatial Variability in Faunal Composition Along Two Middle Devonian Paleoenvironmental Gradients 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 I4627 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 inpaleobiology. 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 bv Cisne. stratigraphically discrete and temporally constrained. ll:~bt,,and otherl: \t>.g., Cisnt and 1<,1h,I9T5; (.'i.i~;(, 1.1 Quantitative analyses of faunal composition among I!)o2; l{al~eandCisnc. 19301invt,lvcd thcctlllec~ig,ni,il,ulk 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 packstonelwackestone to a diverse brachiopodl 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 multzple samples from each locality. 3) Possible non-ran- the hounds 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 uariability 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 0 1994. SEPM (Society for Sedimentary Gealogy) 0883-1351/94/0009-0224/53.00 SPATIAL VARIABlLlTY IN MIDDLE DEVONIAN FAUNAS 225 Cayuga Lde East d Cayuga Lde sessed at both the outcrop and basinal scales along two West oi Cayuga Lake (excepl Fir Tree Anficline) and Fir Tree Anficlins \ A I Middle Devonian, onshore-offshore transects in the Ap- Z Menteth Lime-'--- "'~ palachian Basin. The horizons sampled, the Hills Gulch 5 5 Deep Run St and the Browns Creek beds of the Ludlowville Formation, 20, are stratigraphically discrete and temporally constrained (Fig. 1).These two beds differ somewhat in inferred depositional history and were selected to determine whether their contrasting modes of deposition would impart dif- ferent faunal signatures. Using samples collected from these T beds, several interrelated patterns were investigated: 1) Spatford Shale Mbr. ( Spaford Shale ~bq The spatial scales at which paleoenvironmental gradients mrur*i.mi,r. are identifiable based on faunal composition; 2) The variations in environment that produce differentspatial faunal transitions at various scales (i.e., within-outcrop versus basinal scale); 3) Similarities and differences among spatial patterns exhibited by the faunas of two separate heds. Below, we first discuss the inferred depositional histories and lithologies of the two sampled heds and the meth- odologies for collecting and analyzing data. Then, the an- alytical results are presented. They reveal systematic spatial variations in macrofaunal assemblages on the basinal scale associated with paleoenvironmental gradients in the Appalachian Basin. Compositional transitions from locality to locality generally transcend the level of faunal variability recognized at single outcrops, even when localities are closely-spaced. However, the degree of compositional variahility exhibited at the outcrop scale suggests that, to portray faithfully faunal transitions along the paleoenvironmental gradient, it is essential to collect multiple samples at each locality. At the outcrop scale, where local FIGURE 1 -Generalized stratiaraohic column of the Ludlowville For- microenvironments might be expected to vary only ran- mation (modified from ~a~eret>l.:inpress). Study horizons are noted domly, spatial variahility generally appears to he random, with gray shading. exceut for two localities (one in each bed) that exhibit possible, though admittedly questionable, systematic transitions in faunal compos~tion. 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 graphicltemporal constraint: several event beds have been formation becomes more heavily dominated hy terrigenous identified and correlated across the outcrop belt (Brett et elastics, 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 hiogenic carhonate were deposited in the shallow, suh- 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. 11, 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 elastic wedge (Cooper, 1957; Rickard, concentrations of fossil material at eastern localities. 1981; Brett and Baird, 1985). Near the top of the Browns Creek bed, specimens of In western and central New York, the Middle Devonian Dechenella rowi, a proetid trilobite, have been observed Ludlowville Formation, the third-highest of four forma- positioned with the cephalon subhorizontal and the tho- tions comprising the Hamilton Group, is composed of cal- raxlpygidium oriented at a right angle to the cephalon - 226 LAFFERTY. MILLER. & BRET 1 Western New York Shew n Finger Lakes Trough FIGURE ?-Generalized paleogeographic map of the study area with sampling localities superimposed (modified from Mayer et ai., in press). TABLE 1-Sampling localities of the studied beds. Let- be correlated with the Owasco Siltstone and has been in- ters represent

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