REPLY: 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

We thank Retallack (2011) for his comment on the paper by Davies et al. (2010). Davies et al. (2010) cast doubts on the terrestrial interpretation of trace-fossil bearing strata of the Ordovician Juniata Formation and thus on their applicability to the study of early terrestrial ecosystems. We do not think that Retallack's (2011) comment on this paper dispels these doubts, for the reasons listed herein. If sedimentary facies analysis were as straightforward as inferred by Retallack (2011) in the first paragraph of his comment, then it would be easy to determine, one way or another, the depositional environment of the sedimentary succession at Potters Mills. Although Retallack (2011) implies that such phenomena as “turbidites, hummocky cross-bedding, and flaser and lenticular bedding” are diagnostic of marine conditions, none of them are (e.g., Greenwood and Sherman, 1986; Martin, 2000). We highlight this misapprehension to emphasize that sedimentary structures generated by hydrodynamic processes are rarely diagnostic of particular environments and must be used to this end with great caution. In the case of the Juniata Formation at Potters Mills, as illustrated by Davies et al. (2010), there are no physical sedimentary structures in the succession that are diagnostic of any specific depositional environment, marine or continental. This observation alone casts doubt on studies that have previously offered firm sedimentological interpretations of the Potters Mills succession. Retallack (2011) is correct to point out that better exposed outcrops of parts of the Juniata Formation have permitted interpretations of deposition within braided fluvial systems (Cotter, 1978), but fluvial deposition does not account for the whole formation (e.g., Thompson, 1970; Driese and Foreman, 1991, 1992; Ettensohn, 2004; Davies et al., 2011). Our contention remains that while sedimentological signatures in the Potters Mills section do not permit unequivocal diagnosis of either …