Carbonate ramp to deeper shale-shelf transitions of an Upper Cambrian (Dresbachian) shelf embayment, Nolichucky Formation, southwest Virginia

Abstract

The Nolichucky Formation (0-300 m thick) formed as a transgressive-regressive package on the Cambrian pericratonic shelf in a broad, deep water embayment, bordered to the northeast and southeast by peritidal Honaker and Elbrook carbonates. Lateral facies changes from deep embayment rocks to peritidal carbonates suggest that the shelf embayment was bordered by a gently sloping carbonate ramp. Peritidal facies are cyclic, upward-shallowing stromatolite sequences. These grade seaward into subtidal thick, cross-bedded ooid and oncolitic intraclast-grainstones that pass into subwave base deeper ramp nodular dolomottled ribbon carbonates and rare thin lenses of conglomerate. Ribbon limestones are layers and lenses of trilobite packstone, parallel and wave-rippled laminated quartzose calcisiltite and lime mudstone that occur in stormgenerated fining-upward sequences (1-5 cm thick) and as burrowed units. Deeper embayment facies are green, calcareous shale with open marine biota; storm and tidal-current deposited parallel to hummocky laminated, calcareous siltstone; flat pebble conglomerate and rare paraconglomerate (10-60 cm thick) with clasts of deeper water lithologies (storm-lag, debris-flow, or other subaqueous gravity flow deposits); and shaly packstone/wackestone with trace fossils, glauconite horizons, hardgrounds, and erosion/truncation surfaces. A 15 m thick tongue of cyclic carbonates within the shale package contains subtidal calcareous algal, digitate "stromatolite” bioherms and ooid packstones which suggest a period of shoaling in the embayment. Understanding the genesis of these lithofacies may help interpretation of similar rocks that are widely distributed in North American Cambrian and Ordovician units elsewhere.