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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 12. (December)

First Page: 1919

Last Page: 1919

Title: Cyclic Sedimentation Patterns in Middle Ordovician Trenton Group in Central Pennsylvania: ABSTRACT

Author(s): Mary Anne Gardiner


The carbonate facies of the Middle Ordovician Trenton Group show repetitive sequences of micrite, bioclastic limestone, and siliciclastic shale. Five repetitive patterns based on sedimentologic and paleontologic data are present (in ascending order): (1) biosparite, (2) intrabiosparite, (3) pelsparite, (4) micrite, (5) alternating micrite and shale.

The biosparite is of peritidal origin and is overlain by a stromatolitic cap. The average sequence thickness is 30-40 cm (12-16 in.). The intrabiosparite is a fining-upward sequence that grades to pelsparite with a micrite cap (total thickness averages 16 cm or 6 in.). This sequence is indicative of intershoal or shoaling conditions. Overlying the intrabiosparite sequence is pelsparite grading into a micrite cap. The pelsparite averages 8 cm (3 in.) in thickness, and is of shallow sublittoral origin. Highly burrowed micrite (2-6 cm or 0.8-2.4 in. thick), with a hardground cap, indicates periodic exposure. The upper unit is a deeper, sublittoral sequence of alternating kerogenic micrite and siliciclastic shale, ranging in thickness from 20 to 90 cm (7.9 to 35.4 in.). This pattern indic tes a deepening of the carbonate shelf into a deeper, anoxic basin below wave base.

These sequences are a result of storm deposition as indicated by shell and intraclast lags, by fining-upward trends, and by abrupt contacts between individual sequences. The series of sequences is a result of the decreasing effects of these storms in the deeper water facies.

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