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The AAPG/Datapages Combined Publications Database

CSPG Special Publications

Abstract


Devonian of the World: Proceedings of the 2nd International Symposium on the Devonian System — Memoir 14, Volume I: Regional Syntheses, 1988
Pages 265-275
U.S.A.

Paleogeographic Reconstruction of the Lower Devonian Helderberg Group in the Appalachian Basin

R. Smosna

Abstract

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.


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