科研之友
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摘要
Beaches on which tidal processes play a significant role, as opposed to purely wave-dominated beaches, are little known from the rock record due to a lack of suitable sedimentological criteria and facies models. Cambrian strata of the Cabos Formation in Asturias, Spain, with a wealth of tide-generated sedimentary structures and a close similarity to modern tide-dominated beaches, provide an opportunity to advance our understanding of ancient tidal beaches. Our field-based analysis of the Bres and Taramundi members identifies three lithofacies: Facies 1-simple bedsets of fine sandstone, divided into (1A) sandstone with tabular and wedge-shape geometries and horizontal to low-angle laminae, (1B) sandstone with tabular, wedge-shape, and wave-form geometries and decimeter-scale planar cross-bedding, and (1C) sandstone with a channel form and erosional base; Facies 2-composite bedsets of mudstone and fine sandstone; and Facies 3-intraformational breccias composed of (3A) rip-up clasts, (3B) sandstone clasts, and (3C) skeletal clasts. The numerous tidal structures testify to bed-load transport by unidirectional and bidirectional tidal currents, alternating deposition from bed load and suspended load, emergence during the ebb stage of tidal cycles, tidal scour, and regular fluctuations of tidal-current velocity. In contrast, structures indicative of swash-backwash, oscillating waves, and storms are much less conspicuous. This study argues that the Cabos sandstone and mudstone accumulated on a tide-dominated, wave-influenced beach distinguished by four morphologic zones: a flat sandy supratidal beach (Subfacies 1A), a sandy intertidal beach characterized by small 2-D dunes of a ridge-andrunnel system (Subfacies 1B), cross-shore channels (Subfacies 1C), and the subtidal nearshore with a mix of mud and rippled sand (Facies 2). Rare storm processes on the tidal beach are suggested by the breccias (Facies 3). As increasingly more local examples like the Cabos are recognized in the rock record, sedimentologists can begin to construct a generalized facies model for this largely unexplored sedimentary environment.
- 出版日期2016-12
