Abstract

In the tectonically active Circum-Pacific Belt, thick transgressive sequences are not uncommon, in contrast with their rarity in the Cretaceous of the Western Interior. Thick transgressive sediment packages reflect rapid sedimentation rates but even more rapid rates of relative sea-level rise. A well-exposed example is the Cape Sebastian Sandstone, a 200-m thick, fining-upward sequence representing foreshore to offshore deposition. Progressively increasing depth of deposition is indicated by both physical and biogenic sedimentary structures in the Cape Sebastian Sandstone. Four facies make up the Cape Sebastian Sandstone. The lowest unit is a basal, shelly, boulder conglomerate overlain by trough cross-bedded pebbly sandstone, plane-laminated coarse sandstone, and crudely graded conglomerates. A single type of subvertical trace fossil is locally abundant. These sediments represent beach to nearshore deposition. The middle and thickest part of the formation comprises hummocky-bedded sandstone. divided into a lower hummocky-bedded facies and an upper hummocky-bedded and burrowed facies. Grain size, frequency of pebble lenses, and thickness of hummocky laminae all decrease upward through this part of the sequence. Conversely, burrowed zones, diversity of burrows, plane-laminated zones, plant debris, and symmetrical-ripple preservation increase upward. These sediments record storm-influenced, inner-shelf sedimentation. The uppermost part of the formation consists of alternating laminated, very fine sandstone and progressively thicker, burrowed sandy siltstone. Increased trace-fossil size and number and abundant plant debris characterize these sediments, which represent outer-shelf deposition. Modern examples of the structures described above have been observed on the Oregon and California shelves, supporting the hypothesis that the Cape Sebastian Sandstone represents a transgressive shelf sequence. The same structures have also been described in progradational ("regressive") sequences in the Cretaceous of the Western Interior, where thick transgressive sequences are rare or absent. Evidence for Late Cretaceous faulting in southwestern Oregon supports the proposition that thick transgressive sequences may be deposited in tectonically active regions.

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