82-5 Reinterpretation of tide- and wave-dominated coastal deposits from the Cambrian–Ordovician Los Cabos Group, Asturias, Spain, as formed in a mud-rich, storm-flood-dominated delta

Abstract:

The middle Cambrian to Lower Ordovician Los Cabos Group from Asturias, Spain, constitutes shallow-marine deposits that have been previously interpreted as originated in tide- and wave-dominated coastal subenvironments (e.g. tidal flats, lagoon, beach, shoreface). However, investigations in the area have found evidence of deposition in storm-flood deltas. The goal of this study is to perform a facies analysis of the Los Cabos Group to construct a depositional model based on current advances in deltaic sedimentology. About 5700 m of strata were logged between the towns of Luarca and Cadavedo (Spain), describing sedimentary facies and facies associations. This group consists of mudstone, sandstone, muddy sandstone, and sandy mudstone deposited on shelf, tide- and wave-dominated coasts, tide-dominated, fluvial-influenced coasts, and storm-flood-dominated delta environments. Evidence of storm-flood deltas is extensive and includes: (1) cm- to m-thick, mudstone, sandy mudstone, and muddy sandstone structureless or with normal gradation, banding, load casts and convolute bedding, associated with hummocky cross-stratified sandstone, interpreted as storm-influenced hyperpycnal flows; (2) heterolithic bedding containing low-angle, current-ripple (regular and climbing), wave-ripple (regular and aggrading), and micro-hummocky cross-stratified, muddy sandstone, constituting prodelta deposits, (3) hummocky and swaley cross-stratified muddy sandstone and sandstone and low-angle cross-bedded to parallel-laminated sandstone, characterizing delta-front deposits; and (4) cross-bedded sandstone, heterolithic bedding with low-angle and current-ripple cross-laminated sandstone, and structureless sandy mudstone, representing distributary channels. Characterizing the depositional model of the Los Cabos Group will improve our understanding of the stacking patterns and stratigraphy architecture of storm-flood deltas. Interestingly, the storm-influenced hyperpycnal flows may be analogous to slurry flows reported from the Cretaceous of the North Sea, a rare type of sediment-gravity flows transitional between turbidity and debris flows. Moreover, the fact that these deltas were active in the Cambrian and Ordovician presents the opportunity to analyze storm-flood processes in deltas fed by braided-sheet fluvial systems, before the colonization of land plants.

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