The Campanian Quartz Claystone Conundrum of the African Transform Margin: A Quartz Rich Silty Claystone with Possible Implications for Climatic Conditions in Africa during the Upper Cretaceous

Since 2007, over 40 deepwater wells have been drilled along the African Transform Margin between Sierra Leone and Ghana, a distance of 1,125 km. Many of these wells have encountered a Lower Campanian stratigraphic section up to 200m thick and described, while drilling, as a monotonous, dark grey to black, soft to firm “claystone”. On wireline logs, however, the sequence may be interpreted to be porous sandstone with gamma ray readings ranging from 25 to 60 GAPI units. Permeability, however, is very low.

X-ray diffraction (XRD) analyses suggest that the interval is composed of up to 80% quartz with variable amounts of clay minerals, predominantly smectite, illite and kaolinite. Laser particle size analyses (LPSA) show that this section typically consists of up to 50% clay-sized particles with very fine- to medium-grained silt making up most of the remainder. Very fine- to fine-grained sand makes up less than 4% of the volume. Dual oil based micro-imager (D-OBMI) wireline data indicates rhythmically bedded, varve-like laminae in borehole view.

This “claystone” is common in the deepwater wells studied except where it has been removed by erosional events and replaced by marine slope complexes composed of sand rich turbidite channel systems.

A similar quartz-dominated “claystone” is also found in the Santonian and Turonian stratigraphy of several wells.

These Upper Cretaceous siltstones and mudstones are interpreted to have been deposited in quiet marine settings during third-order sea-level highstands. Paleotectonic and paleoclimatic reconstructions by various authors suggest deposition near the position of the late Cretaceous paleo-equator during greenhouse conditions. However, if these unusual facies are the product of aeolian transportation and subsequent suspension settling on the Cretaceous continental slope, then an arid source terrane is likely and loess deposition is proposed. The laminated nature of these fine-grained deposits further suggests a seasonal driver, with largely wind born material derived from central Africa and transported into the equatorial region by southeasterly winds.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California