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Abstract

Fluvial-deltaic strata of the Upper Cretaceous Ferron Sandstone, Western Interior Seaway, form a clastic wedge consisting of eight short-term stratigraphic cycles. The cycles are arranged consecutively in a seaward-stepping, vertically stacked, and landward-stepping stacking pattern. The stacking pattern is a product of fluctuations in accommodation-to-sediment supply (A/S) regimes described by intermediate-term, base-level cycles.

Each short-term stratigraphic cycle is a progradational/aggradational unit comprising a spectrum of coastal-plain, bay/lagoon/estuary, shoreface, and shelf facies tracts. Sediment volumes and sand-stone:mudstone ratios were measured separately in coastal-plain and shoreface facies tracts in four of the cycles. Total sediment and total sandstone volumes are partitioned differentially into the two facies tracts in a systematic manner that follows the stacking pattern. The total sediment volume and total sandstone in the shoreface facies tract decreases regularly from seaward- to landward-stepping stacking patterns. The proportion of marine-to-nonmarine sandstone also decreases. This demonstrates increasing sediment storage in continental environments during the transition from seaward- to landward-stepping stacking patterns.

Sediment volume partitioning is accompanied by systematic changes in numerous other strati-graphic and sedimentologic attributes which illustrate the two types of facies differentiation. The first type — stratigraphic control on the types of geomorphic elements that occupy a geomorphic environment — is manifest by the transition from fluvial- to wave-dominated deltas in the progression from seaward- to landward-stepping cycles. The second type — a change in degree of preservation of original geomorphic elements — is illustrated by conspicuous differences in the facies that compose the shoreface and coastal-plain facies tracts. Shorefaces of high-accommodation, landward-stepping cycles comprise homogeneous, cannibalized and amalgamated sandstones, whereas shorefaces of low-accommodation, seaward-stepping cycles are lithologically heterogeneous containing diverse facies and well-preserved, original geomorphic elements. Distributary channelbelt sandstones of landward-stepping cycles are composed of high diversity, well-preserved macroforms and bedforms, whereas those of seaward-stepping cycles are composed of strongly cannibalized, amalgamated, low-diversity macroforms and bedforms.

Sediment volume partitioning and facies differentiation are attributed to changing A/S conditions that accompany short- and intermediate-term base-level cycles. The A/S conditions control or influence the position and volume of sediment accumulation, the types of geomorphic elements in an environment, and the proportions and completeness of original geomorphic elements that enter the strati-graphic record.

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