DEBRIS FLOW DEPOSITS OF POSSIBLE ORDOVICIAN AGE ALONG THE WESTERN EDGE OF THE MESOZOIC BASIN IN SOUTH-CENTRAL PENNSYLVANIA

Triassic-age limestone fanglomerates are common to the Mesozoic Basin of south-central Pennsylvania. The alluvial-derived clasts within the fanglomerates range in size from pebble to boulder and are set within a characteristic, finer-grained, reddish-brown mud to silt-size matrix.

A limestone conglomerate in the Fairfield area of Pennsylvania lacks the defining reddish-brown matrix posing the question of whether or not the rocks are of Triassic age. Field examination of the lithologic and bedding characteristics of the conglomerate indicates that the exposure is a series of Ordovician-age, sub-aqueous debris flow deposits.

At least four major cycles are visible along the quarry highwall; each cycle defined by wedges of massive, basal limestone conglomerate draped by carbonate muds and siltstones before grading into a laminite sequence. The variety of rounded to sub-angular limestone, marble, dolostone and chert clasts, occurring within the conglomerate implies a mixed provenance. Similarities of the clasts to Lower to Middle Ordovician carbonate bedrock mapped in the Fairfield and Frederick, Maryland areas establishes a general time frame for the debris flows as well as serving as the source rock. Reaction rims and micro-tectonic structures noted among the carbonate clasts are indicative of post-depositional diagenetic and tectonic activity.

The occurrence of multiple-stacked and offset beds of limestone conglomerate, separated by laminites, infers that there were cycles of debris flows followed by periods of quiescence presumably during periods of onlap/offlap of the Ordovician seaways across the carbonate platform.