Characteristics of coarse‐grained sequences deposited in nearshore, wave‐dominated environments—examples from the Miocene of south‐west Oregon

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ABSTRACT The conglomeratic sandstone of the Sandstone of Floras Lake (Miocene) is a superbly exposed example of a coarse-grained sequence deposited in a high-energy, storm-wave-dominated, shallow-marine environment. The features of the unit suggest deposition on a shoreface in proximity to a fluvial source. Distinguishing characteristics include amalgamated hummocky-stratified sandstone, low-angle cross-stratified conglomerate beds, lenticular conglomerate beds, tabular conglomerate beds, high-angle scours, trough cross-bedded sandstone, organic-rich beds, and burrows of several sizes. These features are interpreted in terms of their depths of deposition, generation by fair weather, storm, and/or flood processes, and as indicators of relative rates of sedimentation. The unit records complex interaction of tectonic, wave, fluvial and tidal processes. Relative depth changes indicated in the sequence probably reflect changes in the balances between sediment supply, basin subsidence, and eustatic sea-level changes. Wave processes are manifest in the presence of structures deposited both above and below fair-weather wave base. Deposition and reworking by chaotic seas associated with local storms and by long-period swell associated with distant storms may be indicated by the interbedding of amalgamated hummocky-stratified sandstone and conglomerate beds. The effects of the lateral migration of a fluvial source with time are reflected by the changing nature and apparent rates of supply of sediments deposited in the nearshore zone. Tidal processes are manifest largely as superimposed features on wave- and current-generated structures. Biogenic structures are an important clue to the rates and frequencies of all these processes. The conglomeratic sandstone was deposited on a high-energy, tectonically active continental margin, similar to the present setting of the southern Oregon coast. It was deposited at varying depths in a subsiding basin during a time of world-wide sea-level rise. The sequence provides an opportunity for the study of coarse-grained nearshore-marine sediments and the delineation of the interaction of various processes during deposition. Detailed sedimentologic and palaeoecologic analysis provides insight into the nature of storm and flood processes that are difficult to observe in the modern environment.

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