Very Early Supratidal Diagenesis in Washover Sediments, Puerto Penasco, Gulf of California, Mexico

Brian E. Lock, Darlene L. Broussard

Puerto Penasco, Sonora, Mexico is built on a macrotidal (tidal range up to 7 m) barrier complex. East of the town, a filled washover channel has been cut off from the sea by dune migration, but has experienced periodic inundation by marine waters--most recently as part of an aquaculture program. The climate is hot [mean August air temperature 30°C (86°F), mean annual rainfall 7.4 cm (2.9 in.)].

Unconsolidated sand samples from core material were dehydrated with alcohol and impregnated with L. R. White resin, an extremely low viscosity agent used primarily by biologists. This resin is cured at 65°C (149°F), a temperature unlikely to significantly modify sensitive mineralogies over the short times involved. The method was extremely successful in preserving highly delicate microstructures such as micrite-filled endolithic algal borings within hollow micrite envelopes.

The surficial sediments, to a depth of about 5 cm, contain halite (principally with fibrous extruded habits) subordinate to fels-arenite sands. The underlying sands, to a depth of about 50 cm (20 in.) are composed predominantly of lenticular gypsum crystals, average diameter 0.5-2 mm (0.02-0.08 in.). Gypsum in the deeper samples is highly corroded, and halite cubes are absent, although the latter are relatively common in shallower samples.

Samples from depths approaching 1 m (3 ft) have abundant calcite microspar cements, preserved calcitic fossils (red algae, for example) and hollow micrite envelopes indicating the dissolution of aragonitic shell fragments. Early cemented sandstone intraclasts are common.

Study of cored material from this sand body reveals a surprisingly complex diagenetic history, probably representing at most a few hundred years. Marine flooding episodes have introduced sulfates and chlorides; freshwater influx, perhaps following infrequent heavy rains, has led to partial dissolution of evaporites and redistributions of carbonate. Long time periods clearly are not a prerequisite for complex diagenetic changes in clastic sediments.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.