Interbedded Alluvium, Tidal Muds and Coarse Littoral Deposits near San Felipe, Baja California--Implications for the Sedimentary Record

D. L. Gautier, C. J. Schenk, J. C. Navarro Fuentes

North of San Felipe, Baja California, recent sedimentary sequences in tide-influenced environments along the Gulf of California include complex interfingering of alluvium, tidal muds, and coarse littoral deposits. In these sediments, facies from different depositional systems are interbedded on a small scale (cm to dm). We dug trenches in tidal flats bounded on the west (landward) by alluvium and on the east by the modern beach ridge; four principal facies were recognized in the trenches. Facies One is a crudely stratified, coarse to medium-grained shelly sand identical to that of the modern beach ridge and associated washover fans. Facies Two consists of reddish tidal muds bioturbated by fiddler crabs. Facies Three consists of stratified but nonbioturbated reddish-brown uds which, like Facies Two, are associated with tidal channels. Facies Four comprises dark gray-green micaceous sands that are laterally extensive and display a distinctive upward progression of stratification types.

The lowermost sands of Facies Four have sharp bases and are characterized by parallel stratification and/or undulatory cross strata. The undulatory strata are similar to hummocky cross strata. These strata are overlain by climbing ripple deposits. Current indicators point seaward. Overlying the ripple deposits are muds containing burrows of marine crustaceans; the burrows descend into or through underlying sands. The parallel-stratified sands probably represent high-energy, upper-flow-regime sheetflood deposits. Undulatory strata may result from draping of parallel strata over irregularities on the muds or may represent the deposits of breaking antidunes that are common during sheetflooding. Climbing ripples reflect waning currents and rapid deposition from suspension, probably due to ponding between the modern and relict beach ridges within the mud flats. Overlying mud deposits represent renewed sedimentation from the tidal regime.

All four facies are in a progradational setting that will probably be incorporated in the sedimentary record. Sequences of parallel to undulatory-stratified sand, with upward-waning energy characteristics and climbing ripple strata, are not common in tidal deposits. If such a vertical succession of sedimentary structures and textures was encountered in limited outcrops or cores, it would require extraordinary care in interpretation to prevent misidentification of facies. Indeed, these interfingering alluvial and tidal deposits display structures and textures similar to storm-generated, density underflow deposits associated with shallow marine shelves.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.