Abstract

The most commonly applied deltaic model in lithogenetic analysis of ancient deposits has been that constructed by major river systems. Perhaps an equally important model is the short-headed stream delta model exemplified by the Pennsylvanian Haymond Formation in the Marathon basin. This model consists of clay, silt-sand couplets in the lower part, and bouldery conglomeratic sand in the upper part. The lower fine-grained deposits represent prodelta and delta front, the upper coarse-grained deposits are the delta plain.

The difference between short-headed stream-delta deposits and their large river-delta equivalents arises in the nature of the delta front—delta plain deposits. In the Haymond Formation, the flysch-like delta-front interval probably indicates proximity to the source terrane in the southeast and sediment influxes in amounts greater than the organisms can rework. The gravelly sand delta-plain deposits indicate that most of the clay and silt common on other delta plains was flushed farther offshore. The small proportion of current-bedded shale and siltstone suggests a largely subaqueous delta plain; however, some subaerial sedimentation is indicated by bleached, root-marked sandstone. In addition, the delta plain was traversed by braided distributaries as evidenced by multiple-channel sandstones. That the channels were choked by a vast assortment of large particles probably transported during periods of greatest runoff is indicated by the basal conglomerates and boulder beds. However, some large boulders may hove been reworked from nearby coastal scree detritus and steep cliffs during intense storms.

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