COMMENT: MARINE INFLUENCE IN THE UPPER ORDOVICIAN JUNIATA FORMATION (POTTERS MILLS, PENNSYLVANIA): IMPLICATIONS FOR THE HISTORY OF LIFE ON LAND: PALAIOS, v. 25, no. 8, p. 527-539, 2010

Abstract

Davies et al. (2010, abstract) propose that, contrary to past studies, “The evidence suggests that the Juniata Formation at Potters Mills was deposited in a marginal marine setting and, as such, no evidence for early life on land can be inferred from its strata.” The new evidence they present, not only from the Juniata Formation at Potters Mills, but from other localities in Pennsylvania, is a facies analysis and description of sedimentary structures. This is a welcome approach to the problem, because there are distinctive marine sedimentary structures and facies such as turbidites, hummocky cross-bedding, and flaser and lenticular bedding. However, this promising approach failed because neither these nor any diagnostic marine sedimentary structure is described from the Juniata Formation by these or prior authors. The sole plausible exception is a single case of reversing cross stratification at Waggoners Gap, which is inadequately exposed and qualified by Davies et al. (2010) as a possible artifact of outcrop orientation. The sedimentary structures and facies of the Juniata Formation have long been interpreted as formed in braided streams (Cotter, 1978), thus explaining the lack of lateral accretion deposits formed by meandering streams. “Under other circumstances, particularly given the nondiagnostic nature of the sedimentary facies, it would be standard practice to assume significant marine influence in the Juniata Formation based on the diverse and abundant trace fossils,” write Davies et al. (2010, p. 535), although they make an exception for repichnia (surface tracks and trails). “The lack of evidence for repichnial trackways in the Juniata sets it apart from all other early Paleozoic, terrestrial trace-fossil-bearing successions described to date” (Davies et al., 2010, p. 535). This statement is directly contradicted by their own illustrations of such fossils from the Juniata Formation (Davies et al., 2010, fig. 5N) labeled as “repichnial crossing …