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-.. <br />~~ <br />_} <br />The complexity of the Upper Williams Fork formation is due to the depo- <br />sitional environment in which it formed. The formative environment is <br />hypothesized as having been an eastward prograding deltaic plain along the_ <br />intercretaceous seaway during a period of episodic regression of the sea. <br />Intertongued marine and non-marine sediments and extensive coal forming <br />environments are accounted for in this theory. <br />As the sea regressed to the east, the land surface followed. Deltas built <br />forward depositing prodelta clays and silts grading to coarser sediments <br />toward river mouths. At some point the river periodically shifted and began <br />this depositional pattern in an adjacent area. When this shift occurred, <br />the previously deposited delta lobe was cut off from its supply of sediments <br />and then compacted. The sea then transgressed and covered the lobe. Even- <br />tually the river channel shifted again and the cycle repeated and built a <br />new lobe, burying the previous lobe and causing the land surface to progress <br />eastward. <br />Associated with the prograding land surface were a number of depositional <br />environments. Grading from continental to marine, these included piedmont, <br />floodplain, mainland beach, lagoon, barrier beach (island) and near shore <br />marine sediments. Coal is thought to have formed as the lagoons filled, <br />forming swamps which reached dimensions of 40 miles wide by 200 to 300 miles <br />long. This long sequence was followed by basinal subsidence which would <br />cause a transgression of the sea, destroying the coal forming environment. <br />The depositional processes caused the coarsest grained members (sandstone) <br />which are the delta front sands to be the most lenticular and discontinuous. <br />2-354 <br />