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Alluvial and Coastal Conglomerates: Their Significant Features and Some Comments on Gravelly Mass-Flow Deposits
Conglomerates originating in coastal environments represent mainly beachface, shoreface, fan-deltaic or deltaic mouth bar, and Gilbert-type delta sequences. They show structures, textures and other features created mainly by the varied influence of waves and fluvial output in the shallow marine setting. Transitional, alluvial/marine systems show a broad range of facies characteristics and sequences, and these are discussed in detail.
Conglomerates originating in alluvial environments comprise mainly braided stream and mass flow sequences. The former include regular braided river and fan (distributary) channel deposits, and show textures and structures which vary greatly with source and climatic setting. Braided stream sequences commonly show an upward fining motif, due to falling flood stage or to gradual abandonment of alluvial tracts. Mass flow conglomerates originate from a variety of debris flows in subaerial settings, but fluidal gravelly flows (like many ‘sheetfloods’ or ‘streamfloods’) may also be important, and they often become prominent subaqueously (high-density gravelly turbidites). In both instances, the deposits show remarkably varied texture, structure, and fabric. Subaerial flows are often considerably transformed when passing into water. A review of diagnostic features and facies sequences is presented.
When interpreting the emplacement mechanics of mass flow conglomerates, particular effort must be made to extract maximum information from the individual bed characteristics. We illustrate with examples that even such basic data as bed thickness and maximum clast size may serve as a valuable source for some genetic inferences.
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