Evaluating Tectonic Controls on Paleovalleys with a Structurally Deformed Basement

Differential erodability of underlaying strata can play a large role in defining the morphology and spatial distribution of paleovalleys; however, distinguishing the effects of an inherited structurally deformed basement from that of active structural deformation can be difficult. Here we propose a set of criteria to distinguish the effects of inherited structural fabric compared with the addition of basin tilting and/or localized deformation. An active tectonic signal may be determined by evaluating (1) if there is a regional asymmetry in the alluvial and bedrocks channels and valleys and whether those asymmetry trends match one another and the local or regional structural trends, (2) if the regional erosion surfaces within the valleys show trends within the depth structure map or ispoachs between surfaces that match the local structural trends, and (3) if the planform morphology changes immediately adjacent to and/or over possible folds or faults. These criteria have been developed for the Murrells Inlet paleovalley offshore of South Carolina, a well imaged paleovalley on the continental shelf, interpreted to be one of seven paleovalleys related to the piedmont draining Pee Dee River since the Pliocene. This Pleistocene paleovalley is located on the US Atlantic passive margin; and despite this tectonic setting, previous interpretations suggest that the Pee Dee River paleovalleys have migrated south over time. This migration is thought to be the result of beach ridge deflection and uplift along the Cape Fear Arch, which has induced a basin tilt to the southeast. Additionally, the Cretaceous and Paleocene siliciclastic and carbonate strata that are incised by the paleovalley are structurally deformed, with folds and faults of unknown timing and origin. This case study provides the opportunity to assess the practicality of applying these criteria to distinguish active tectonism from inherited topography; and by doing so, has wider implications for using mixed bedrock-alluvial paleovalley morphology as tectonic indicators on passive margins.