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4.1.3 Deposition of Surface Mineable Coal Beds <br />` The complexity of the Upper Williams Fork Formation is due [o the <br />depositional environment in which it formed. The formative envi- <br />ronment is hypothesized as having been an eastward prograding <br />deltaic plain along the intercretaceous seaway during a period of <br />episodic regression of the sea. Intertongued marine and <br />non-marine sediments and extensive coal Forming environments are <br />accounted for in this theory. <br />As the sea regressed to the east, the land surface Followed. <br />Deltas built forward depositing prodelta clays and silts grading <br />to coarser sediments towards river mouths. At some point the <br />river periodically shifted and began this depositional pattern in <br />~~ an aujacent area. When [his shift occurred, the 'previously <br />deposited delta lobe was cut off from it's supply of sediments and <br />[hen compacted. The sea [hen transgressed and covered the lobe. <br />Eventually the river channel shifted again and [he cycle repeated <br />and built a new lobe, burying the previous lobe and causing the <br />land surface [o progress eastward. <br />Associated with the prograding land surface were a number of depo- <br />sitional environments. Grading from continental to marine, these <br />included piedmont, flood plain, mainland beach, lagoon, barrier <br />beach (island) and near shore marine sediments. Coal is thought <br />to have formed as [he lagoons filled, forming swamps which <br />~) reached <br />-26- <br />-~ <br />