Abstract

Graded beds may be produced by deposit-feeding organisms in intertidal and shallow subtidal environments. Barnstable Harbor, Cape Cod, Massachusetts, contains intertidal populations of Clymenella torquata , a sedentary polychaete. Clymenella selectively ingests specific size fractions of sediment at a depth of 10-30 cm within the tidal flats. Sand and silt are transported vertically through the gut and egested as uncompacted feces on the sediment surface. Size fractions too coarse for ingestion ( > 1.0 mm) are concentrated at the lower end of worm tubes producing a well graded stratum. Laboratory measurements indicate that Clymenella is capable of displacing 87 liters of sediment per m 2 per year at the mean annual water temperature of Barnstable Harbor (11 degrees C). The thickness of a graded bed, limited by feeding depth, is less than 30 cm. Maximum and average grain size decrease in regular fashion from the base to the top of the bed. Sorting is poor and sigma phi values vary irregularly in the bed. The size distribution curves are strongly to moderately fine-skewed in samples at the base of beds; curves become more nearly symmetrical at the top of graded beds. Biogenically graded beds may be distinguished from physically deposited graded beds by textural properties, associated sedimentary structures, geometry of the deposits, and lithology of interfingering facies. The best development of biogenic graded bedding occurs in sites of slow physical sediment accretion and relatively weak bottom currents.

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