Dep_Systems2 - A typical delta cycle. A new delta lobe...

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Unformatted text preview: A typical delta cycle. A new delta lobe forms a “bud” ( a ) that enlarges ( b ). It is eventually abandoned ( c ) as the focus of deposition goes somewhere else, and the lobe “decays” ( d ) by subsidence. About 300 million years ago (late Paleozoic) the Appalachian Mountains extended through Arkansas, Oklahoma, and Texas as the Ouachita Mountains. What is West Texas today was occupied by a sea. Streams drained westward from the mountains, a direction opposite to stream flow today. Streams deposited fluvial sediments, and deltas that prograded westward, filling the sea basin with clastic sediment. In places, very deep water prevailed in the seaway until near the end of basin-filling. A block diagram illustrates several components of the delta environment. Coarse bedload (yellow) is confined to distributary channels where it is deposited as elongate “barfinger” sand. Natural levees (orange) comprise the banks of streams. Vast backswamps (blue) are filled with vegetation. In this diagram of a sequence of strata (with large vertical exaggeration) we see channel sand, and backswamps in which mudrock (shale) was deposited. Eventually a delta lobe would be abandoned and with continued subsidence, the sea invaded the area where it deposited limestone (blue) of biochemical origin. An E-W cross section across north-central Texas (see the index map) shows systems of fluvial and delta deposits prograding toward the west. Each higher (later) sequence extends westward farther over the top, and rolls down into what had been deep water in late Paleozoic time. Let’s consider backswamp deposits in a delta. In late Paleozoic time the U. S. Midwest contained sedimentary basins that became filled with delta deposits. Backswamp organic material was buried and transformed into coal. The following image pictures the situation of the Gulf Coastal Plain (yellow, brown) in early Cenozoic time. In Eocene time the shore of the Gulf of Mexico would have passed through Dallas-Fort Worth, Austin, and San...
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This note was uploaded on 09/14/2009 for the course GEO 401 taught by Professor Lassiter during the Spring '08 term at University of Texas.

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Dep_Systems2 - A typical delta cycle. A new delta lobe...

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