Carbonate ramp‐to‐deeper shale shelf transitions of an Upper Cambrian intrashelf basin, Nolichucky Formation, Southwest Virginia Appalachians

Abstract

ABSTRACTThe Nolichucky Formation (0–300 m thick) formed on the Cambrian pericratonic shelf in a shallow intrashelf basin bordered along strike and toward the regional shelf edge by shallow water carbonates and by nearshore clastics toward the craton. Lateral facies changes from shallow basinal rocks to peritidal carbonates suggest that the intrashelf basin was bordered by a gently sloping carbonate ramp.Peritidal facies of the regional shelf are cyclic, upward‐shallowing stromatolitic carbonates. These grade toward the intrashelf basin into shallow ramp, cross‐bedded, ooid and oncolitic, intraclast grain‐stones that pass downslope into deeper ramp, subwave base, ribbon carbonates and thin limestone conglomerate. Ribbon limestones are layers and lenses of trilobite packstone, parallel and wave‐ripple‐laminated, quartzose calcisiltite, and lime mudstone arranged in storm‐generated, fining upward sequences (1–5 cm thick) that may be burrowed. Shallow basin facies are storm generated, upward coarsening and upward fining sequences of green, calcareous shale with open marine biota; parallel to hummocky laminated calcareous siltstone; and intraformational flat pebble conglomerate. There are also rare debris‐flow paraconglomerate (10–60 cm thick) and shaly packstone/wackestone with trace fossils, glauconite horizons and erosional surfaces/hardgrounds. A 15‐m thick tongue of cyclic carbonates within the shale package contains subtidal digitate algal bioherms which developed during a period of shoaling in the basin.Understanding the Nolichucky facies within a ramp to intrashelf basin model provides a framework for understanding similar facies which are widely distributed in the Lower Palaeozoic elsewhere. The study demonstrates the widespread effects of storm processes on pericratonic shelf sedimentation. Finally, recognition of shallow basins located on pericratonic shelves is important because such basins influence the distribution of facies and reservoir rocks, whose trends may be unrelated to regional shelf‐edge trends.