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Wave/Tide Dominated Systems
Depositional Features of Late Miocene, Marine Cross-Bedded Conglomerates, California
Cross-bedded, sandy conglomerate and pebbly coarse sandstone form the dominant facies of the basal part of the Santa Margarita Sandstone in the Santa Cruz Mountains of central California. Fossils in this unit of late middle and early late Miocene age, as well as its geometry, indicate deposition in a shallow, northeast-trending seaway connecting the Pacific Ocean with an interior sea. Cross-stratified sets, up to 8 m thick, are interbedded with conglomeratic lag deposits up to 1.3 m thick. Foresets are defined by variations in conglomerate concentration or by alternating conglomerate and sandstone layers. Most of the pebbles are matrix-supported. The conglomeratic foresets typically dip at angles of less than 20°. Some conglomerate layers overlie reactivation surfaces that separate sandy foresets that dip at angle-of-repose (31°). Intense bioturbation disrupts many of the pebbly foresets.
The lithological character of the basal part of the Santa Margarita Sandstone changes vertically. The coarseness and abundance of conglomerate diminish upwards and the geometry of the cross-bedded units changes from trough to tabular in style. Concomitant with these changes, the foreset inclination direction reverses from northeasterly in the lower, trough units to southwesterly in the overlying tabular units. The lateral array of paleocurrent directions also shows a consistent trend: to the northeast along the southeastern flank of the seaway and to the southwest in the central and northwestern parts. The size and direction of cross-stratification and the abundance of reactivation surfaces indicate that the seaway was swept by strong tidal flow. The trough cross-beds reflect the passage of lunate or sinuous-crested dunes; the tabular units, straight- to sinuous-crested sand waves. The bedform migration was sufficiently slow or infrequent to permit intensive infaunal reworking of the substrata. The vertical sequence observed in the basal part of the Santa Margarita Sandstone appears to record a relative rise in sea level during deposition.
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