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Most studies of early Paleozoic eustatic variations have focused on carbonate platform sequences. Platform-margin deposits have received little attention. The Onyx Cave member of the allochthonous Hamburg klippe sequence of eastern Pennsylvania was deposited at the base of the North American carbonate platform in Late Cambrian time and emplaced on the platform during Middle Ordovician crustal convergence. It consists of (1) thick to very thick quartzose limestone beds, (2) very thick carbonate clast conglomerate beds, and (3) thin to thick laminated calcilutite beds. These deposits are arranged in thinning- and fining-upward cycles identical to channel abandonment sequences documented from clastic submarine mid-fan areas. A particularly interesting feature of the Onyx Cav is the abundance of rounded and well-sorted quartz and minor but conspicuous K-feldspar grains. The applicability of deep-sea fan models to the Onyx Cave member and the lack of mud and slump deposits are consistent with sedimentation on a canyon-fed (point source) carbonate submarine fan rather than the more typical Bahamian carbonate-slope turbidite system (line source). The abundance of well-sorted and rounded quartz sand within the Onyx Cave records a basinward migration of nearshore sediments across the platform toward the head of a submarine canyon where it was funneled into the deep sea. This scenario accords well with investigations of platform sequences that have proposed a Late Cambrian regression, and reinforces the important dependence of deep-sea clastic sedimentation on eus atic variations in sea level.
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