Abstract

X-radiography and scanning electron microscope analyses of shales from diverse sedimentary environments reveal wide variations in macro-and microfabrics attributed to the degree of bioturbation. These fabrics provide clues about the physical condition of the sediment substrate at the time of clay deposition and the relationship between bioturbation activity and the sedimentary environment. Organic-rich shales from anoxic environments are unbioturbated and characterized by fine lamination and a microfabric of parallel clay flakes. Two fabric types were found in shales formed under oxic conditions--highly bioturbated and indistinctly bedded. Fabric in shales subjected to considerable bioturbation is recognized by a homogeneous, gray tone in an X-radiograph and by a random microfabric in a scanning electron micrograph. This fabric is also very similar to mudstones formed from lithified flocculated clay; however, it does not possess the stepped domains or stepped cardhouse fabric of the latter. Indistinctly bedded shale possesses remnant original lamination and preferred particle orientation indicative of less intense bioturbation. The fabric types are useful in determining whether the sediment substrate was a soupground, softground, or firmground, as well as indicating the amount and intensity of biogenic activity during sediment deposition.

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