ABSTRACT

The Late Silurian-Early Devonian Helderberg Group, central Appalachians, is a sequence of mixed siliciclastic-carbonate sediments that was deposited during relative tectonic quiescence on a ramp that was built out from low-relief tectonic highlands bordering the eastern side of the Appalachian Basin. The Helderberg Group is a third-order Vail cycle that lasted about 8-10 m.y. Three transgressive-regressive sequences are also recognized. Each sequence was deposited over 2-3 m.y.; subsidence rates during deposition were about 1-2 cm/1,000 yr. Lithofacies patterns resulted from eustatic sea-level changes, differential basin subsidence, and varying sedimentation rates.

Skeletal grainstone/rudstone formed fringing skeletal banks that developed during regression and prograded away from the eastern side of the basin at rates of 35-50 m/1,000 yr. Quartz sandstone tongues in the Helderberg Group probably were reworked from underlying Silurian sandstone and were introduced to the basin by rivers that nearly coincided with ancestral Silurian drainage systems. Siliciclastic influx was never great enough to shut carbonate production off completely. Thick Middle Devonian siliciclastic sediments buried the Helderberg Group and updip subaerial exposures accompanying the onset of the Acadian Orogeny.