Abstract

The Richland Formation is exposed on both limbs of the Lebanon Valley nappe in southeastern Pennsylvania. The nappe probably contains the farthest transported carbonate platform rocks of the Cambro-Ordovician continental margin sequence. Intertidal to deep subtidal environments are represented. Intertidal deposits include cryptalgalaminates and tabular-laminated dolomites. Ooid grainstones, in part altered to coarsely crystalline dolomites, are shoal or beach deposits. Subtidal deposits include mudstones, peloid packstone/wackestones, thrombolites, and domal stromatolites. Ribbon carbonates were probably deposited in an open-platform setting below fair-weather wave base. Lithofacies occur in cyclic peritidal, subtidal, and thrombolitic facies associations. Facies sequences on each limb are reciprocally related. Upper-limb rocks occupied a more seaward platform position. The upper limb features a 180-m-thick cyclic peritidal sequence followed by 320 m of deep to shallow subtidal deposits. On the lower limb a 300-m-thick cyclic subtidal sequence is succeeded by 100 m of cyclic peritidal deposits. The contrasting facies successions on each limb imply two major depositional phases. Phase one featured a shelf system with a broad shoal complex (upper limb) situated in front of or beside lagoon or embayment deposits (lower limb). In the second phase, deep subtidal conditions spread across the area of the upper limb while cyclic peritidal facies were established in the lower-limb setting. The upper-limb shoal complex probably was incipiently drowned during a transgression that culminated in a late Cambrian-early Ordovician sea-level highstand.

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