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Abstract

The Niobrara Formation of the Western Interior consists of 100 to 250 m of interbedded chalks and calcareous shales. The formation is divided into two members: (1) a thin, basal, dominantly chalk-bearing sequence, the Fort Hays Limestone Member, and (2) an upper, thick, calcareous shale unit, the Smoky Hill Chalk Member. Both units represent widespread pelagic to hemipelagic sedimentation in an epicontinental seaway during a relative highstand of sea level. During this highstand, the shoreline migrated westward and large areas of the Western Interior were covered by relatively deep water and received little terrigenous influx. Productivity of nanno- and microfossils was sufficiently great to yield moderately thick sequences of regionally homogeneous chalk and marl. These strata are very fine grained, have high primary porosity, and contain few macrofosstls other than bivalves.

Sedimentary structures in the Niobrara consist mainly of laminations, fecal pellets, and burrows. Chalk-shale depositional cycles are found at scales ranging from millimeters to tens of meters, Cyclic sedimentation may have been related to variations in climatic patterns which, in turn, influenced terrigenous sediment influx and (or) biological productivity in the region. Climatic fluctuations probably influenced the salinity of the surface waters in the Western Interior seaway by altering circulation patterns and basinal water turnover. This resulted in periods of dysoxic and even anoxic bottom-water conditions within the seaway. Stagnation events are reflected by intervals with virtually no benthic fauna or burrows and with good preservation of millimeter-scale laminations and abundant organic matter.

The primary properties of the chalks and calcareous shales of the Niobrara have been greatly modified by burial diagenesis. Increased burial brought about rapid reduction of porosity by a combination of mechanical and chemical compaction and associated calcite overgrowth cementation. Authigenic pyrite is widespread in association with organic matter. Clay minerals of detritai origin and altered voicanogenic material originally underwent transformation from disordered, predominantly smectitic mixed-layer clays to ordered, predominantly illitic mixed-layer clays. During diagenetic alteration, brittle chalks and calcareous shales were deformed and significant fracturing occurred. Finally, thermal maturation associated with burial led to hydrocarbon generation from the organic-carbon-rich chalks and calcareous shales of the Niobrara. In areas of shallow burial, formation of biogenic methane was widespread and has proven to be economically important.

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