Abstract

Graded bedding is common in intertidal sediments of Mugu Lagoon, California. This bedding is different from other graded sequences which have been described in that the coarsest size-grade at the top of the bed is the same size as the coarsest size-grade at the base; this size simply becomes less common upwards. The grading is accomplished by burrowing organisms which mix basal sand of the lagoon with tidal flat and salt marsh mud naturally deposited over it. Dense marsh growth inhibits burrowers, causing mixing to diminish and finally to cease as the marsh becomes well-established. The graded sequence then is complete. Biologic energy can give a completely different character to sediments initially deposited by physical processes. In a unit of time, at least 10 to 100 times more sediment is placed on the surface by burrowing animals than is added by tidal currents and other transportational agents in most sedimentary environments of Mugu Lagoon. A rapid sedimentation rate is indicated by a radiocarbon date of about 100 years B. P. on Pecten shells found 75 cm beneath a graded sedimentary sequence. This date also gives an estimate of the time that it took to develop the graded bed, and the rapidity with which the marsh has expanded over open lagoon sediments.

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