Evaluation of the multiple origins of thin-bedded deep-water slope sandstones : El Rosario Formation (Upper Cretaceous - Paleocene) Baja California, Mexico
Ochoa Rodriguez, Jesus Armando.
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One dilemma in sedimentology is that multiple depositional processes can produce similar features while one formative process can generate multiple patterns. A comparison of (1) depositional energy trends from grain size and primary structures, (2) placement within a stratigraphic hierarchy, (3) ichnofacies type, diversity and distribution, and (4) sedimentary body type and associated architectural changes are used to assess the causal mechanism. Variations of these attributes reflect flow initiation processes (flood vs. failure), depositional processes (flow stripping, overspilling or bottom current reworking), and preservation (thin-beds bounded by erosional channels). Cretaceous and Tertiary outcrops in the Mesa San Carlos area expose four different thin-bedded sandstone types (TBS). This document presents a matrix of the most important attributes used to recognize them: (1) hyperpycnite successions (4-15m thick) are interbedded with slope mudstone deposits that together form tabular (85m thick; >1km wide) successions, with sandstone channels and scours common at the base and mass transport deposits present at the top, (2) Wedge-shaped TBS turbidites that flank and confine multistory channelbelts up to 90m thick that thin and pinch out within 500m of interdigitated but stacked conglomerate channels, (3) TBS turbidites separating channel bodies form 25m-thick and 230m-wide preserved remnants, and (4) TBS contourites comprising <3m wide sandstone lenses amalgamated laterally to form tabular bedsets. Paleocurrent indicators change from unidirectional offshore during hyperpycnal flow to slope parallel flow during waning energy conditions of this mudstone-rich cycle. The matrix allows for a sedimentological hierarchy that describes the spatial and temporal organization of hydrodynamic facies that link together to form sedimentation units recording changes in the longitudinal structure of the flow. Other results include the field- and lab-based mudstone analysis that allows recognition between hemipelagic and pelagic mudstones; the revision of the regional stratigraphy for the El Rosario Formation, and an energy matrix for hyperpycnite channels and scour deposits recording the downstream and vertical changes in sedimentary bodies, grain size and primary structures.