Multiple Origins of Thin-Bedded Slope Turbidites: El Rosario Formation, Upper Cretaceous - Danian, Baja California, Mexico

Jesus Ochoa
Montana State University, Earth Sciences Department Bozeman, Montana
[email protected]

Thinly interbedded sandstone and mudstone in erosionally to depositionally confined slope valleys are interdigitated with conglomerate channel fills and form meter-scale units encased within a 300m-thick mudstone succession separating slope-valley from underlying canyon fill deposits.
Thin-bedded deposits within the valley fill form up to 25m thick and 230m wide wedge shape bodies consisting of 25–83cm thick, fine-medium, moderate-well sorted, subangular sandstone interbedded with 3-10cm mudstone. Sandstone beds thin laterally from 83 to 5cm over 35m, contain climbing unidirectional ripple cross-lamination, plane to wavy laminations commonly bracket ripples, mudstone thicken laterally. Horizontal sand-filled burrows are present at sandstone bed tops. Thin-bedded deposits flanking and confining multistory slope valley channels form up to 80m thick successions that thin and pinch out over 500m; these consist of 2-15cm thick fine sandstone beds separated by 1-12cm thick mudstone interbeds. Sandstone beds show the most conspicuous lateral grain size changes and contain parallel laminations and climbing ripple cross stratification. Thick slope mudstones encase 4-11m thick thin-bedded deposits that form a tabular 130m-thick and >1 km wide successions. Very fine to fine, well sorted, subrounded sandstone forms 5-25cm thick beds showing little lateral change over 40m with plane to wavy bedded facies most common. Low diverse trace fossil assemblage in both sandstone and mudstone.
Temporal and spatial controls on these three thin-bedded facies assemblage include lateral gradient changes related to levee height; preservation control internal levee thickness, and sedimentation from river initiation flows for the more continuous non-associated slope valley deposits.

AAPG Search and Discovery Article #90070 © 2007 AAPG Foundation Grants in Aid