Origin of Smectite Cement in Miocene-Age Sands of Deep Gulf of Mexico Basin

John D. Whynot, Thomas T. Tieh

Miocene-age sands from Deep Sea Drilling Project (DSDP) Site 90, located on the Sigsbee Abyssal Plain, are cemented by authigenic smectite. These sands are medium to coarse-grained and contain abundant volcanic and plutonic rock fragments. In contrast, similar Miocene-age sands from DSDP Site 91, also located on the Sigsbee Abyssal Plain, do not show development of smectite cement. Smectite cement development was controlled by early post-depositional fluid flow, as the cemented Site 90 and uncemented Site 91 sands are similar in age, texture, composition, burial depth and initial pore-water composition.

Cemented sands at Site 90 are thin (< 1 m thick) and bounded by low-permeability interlaminated fine-grained sand and mud that acted to reduce the rate of fluid flow through the coarser grained sand. Reduction in fluid flow allowed the concentrations of alumino-silicate polymers, alkali, and alkali-earth cations, supplied by the alteration and dissolution of volcanic rock fragments, to reach levels necessary for the precipitation of smectite. Uncemented sands at Site 91 are thicker (> 1 m thick) and not bounded by interlaminated sand and mud. Thus, fluid flow through sands at Site 91 was not impeded, and dissolution products were removed before they could reach concentrations necessary to precipitate cements.

A comparison of the authigenic and detrital phases that appear to be stable in this system, with phase relationships in the system K₂O-Na₂O-SiO₂-Al₂O₃-H₂O, suggests that the pore-water composition at the time of smectite precipitation was similar to that of normal seawater.

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