The fluvial-to-marine transition within the post-rift Lower Cambrian Hardyston Formation, Eastern Pennsylvania, USA

Abstract

Abstract Identification of fluvial-to-marine transitions is often problematic in Neoproterozoic and Cambrian strata due to the absence of diagnostic, physical, and/or biogenic sedimentary structures. This study highlights the identification of a fluvial-to-marine transition in a post-rift setting. Siliciclastic sediments of the Early Cambrian Hardyston Formation, in eastern Pennsylvania, are assigned into four facies associations (FA A–D), within which a fluvial-to-marine transition can be delimited. The lowermost subdivision, FA A, consists typically of thick-bedded, fining-upward beds of structureless, pebbly conglomerate that grade into structureless sandstone. Within the uppermost FA A, structureless sandstone is replaced by trough and planar, cross-stratified sandstone. FA A is best interpreted as deposits of a bedload-dominated, braided fluvial system. FA B consists of densely packed Skolithos burrows, thick-bedded, normally graded sandstone with rare poorly preserved tabular-planar and trough cross-beds near its upper boundary. Sedimentary structures in FA B are analogous to those found in a shoreface setting. Normally graded beds reflect deposition from waning storm-induced flow, with structured sandstone recording possible fair-weather or post-storm reworking. The fluvial–marine transition lies at the abrupt change from FA A to FA B and is identified by vertical changes from: (1) feldspathic to quartz sandstone, (2) fluvial bar to shoreface storm deposits, and (3) nonbioturbated to bioturbated strata. Additionally, heavy minerals are concentrated along the boundary. FA C is composed of trough cross-stratified conglomerate and sandstone with paleocurrents interpreted as shore-parallel and offshore-directed marine currents. FA C represents a marginal-marine setting reflecting a gradation from FA B to FA D, including characteristics of both Skolithos of FA B and a number of sedimentary structures indicative of FA D. FA D consists of offshore-directed, tabular-planar and trough cross-stratified quartz sandstone separated by thin siltstone–mudstone beds. Some tabular-planar cross-stratified beds contain obliquely oriented ripples on siltstone-draped pause planes. Preserved sinuous-crested dunes with superimposed current ripples cap some cross-bed cosets. The presence of tidal currents is inferred to be the mechanism responsible for the formation of FA D in a probable subtidal marine setting. The Hardyston Formation accumulated in response to thermal or jerky subsidence. Aggradational fluvial deposits developed as a result of the initial Sauk trangression. The fluvial–marine transition most probably records the shoreface retreat unconformity.