About This Item
- Full TextFull Text(subscription required)
- Pay-Per-View PurchasePay-Per-View
Purchase Options Explain
Share This Item
Paleogeographic Reconstruction of the Lower Devonian Helderberg Group in the Appalachian Basin
Although dominated by limestone the Helderberg Group is a unit of diverse lithologies, complicated by horizontal and vertical facies changes, diachronous formations, unconformities and cyclic sequences.
The Helderberg sea occupied an intracratonic basin whose central axis passed through West Virginia and Pennsylvania. Subsidence was gradual (1 cm/1000 yrs), controlled by subtle movements along basement faults. Positive but inactive tectonic elements surrounded the sea, though only a landmass to the east supplied terrigenous sediment. Connections to the open ocean apparently existed on the north, south and east.
Carbonate sedimentation took place on gently sloping rises (10–15 cm/km) along both sides of the basin axis. The upper rise contained peritidal dolomitic micrites and stromatoporoid biostromes of restricted lagoons. Crinoidal sand shoals, coral-stromatoporoid buildups and mud mounds developed on the middle rise. On the lower rise accumulated cherty biomicrites, grading into cherty, calcareous shales of the basin centre (maximum water depth 45–60 m). Quartzose sandstones formed as barrier beaches during brief periods of terrigenous influx.
The group was deposited during a succession of two large scale cycles which resulted from eustatic sea-level changes. Superimposed on these slow transgressive-regressive cycles (80 m thick) were numerous shallowing upward sequences (1 to 9 m) produced by rapid sediment aggradation.
Pay-Per-View Purchase Options
The article is available through a document delivery service. Explain these Purchase Options.
|Protected Document: $10|
|Internal PDF Document: $14|
|Open PDF Document: $24|