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In the tectonically active Circum-Pacific belt, thick retrogradational ("transgressive") sequences are not uncommon, in contrast with the Cretaceous of the Western Interior. Retrogradational sediment packages reflect rapid sedimentation rates but even more rapid rates of relative sea-level rise. A well-exposed and convincingly documented example is the Cape Sebastian Sandstone, a 250-m-thick, fining-upward sequence representing foreshore to offshore accumulation.
Progressively increasing depth of deposition is reflected in both physical and biogenic sedimentary structures in the Cape Sebastian Sandstone. The basal, shelly boulder conglomerate is overlain by trough-cross-bedded pebbly sandstones, plane-laminated coarse-grained sandstones, and graded conglomerates. A single, subvertical trace-fossil type is locally abundant. These sediments represent foreshore to nearshore depositional environments.
The middle and bulk of the formation comprises hummocky cross-stratified sandstones. Grain size, frequency of pebble lenses, and thickness of hummocky laminae decrease upward. Burrowed zones, diversity of burrows, plane-laminated zones, plant debris, and oscillation-ripple preservation increase upward. These sediments reflect storm-influenced, inner-shelf sedimentation.
The uppermost part of the formation consists of alternating laminated, fine-grained sandstones and progressively thicker, burrowed sandy siltstones. Increased trace-fossil size and diversity, as well as abundant plant debris, characterize these sediments, which represent an outer-shelf sedimentary environment.
Modern examples of the described structures have been observed off the Oregon and California coasts, corroborating the hypothesis that the Cape Sebastian Sandstone represents a retrogradational shelf sequence. Also, evidence for Late Cretaceous faulting in southwestern Oregon supports the proposition that thick, retrogradational sequences may be deposited in tectonically active regions.
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