Abstract

The Lower Cretaceous arkoses of the Angola offshore basin have been extensively diagenetically altered. The results of a multidisciplinary study which included petrography, K/Ar dating and 18 O/ 16 O and 13 C/ 12 C ratio measurements combined with fluid inclusion data provide information about the timing and temperatures of formation of the diagenetic cements as well as the nature and origin of the diagenetic fluids. During burial, K-feldspar overgrowths formed at around 100 Ma, followed by discrete development of quartz overgrowths. Locally, both are preceded by chlorite rim cement. Then, chronologically, dolomite, calcite and clay cements precipitated. The microcrystalline clays are an IMII-ordered illite/smectite of Eocene age (50 Ma) and/or a pure illite of Miocene age (16 Ma). Both are commonly mixed with a Fe-rich chlorite. Fluid inclusion data (Walgenwitz and Pagel 1984; Walgenwitz et al. 1985, 1989) from the K-feldspar, quartz and dolomite cements indicate that diagenetic fluids were high temperature (110-165 degrees C) brines (24 wt. % eq. NaCl). Estimated temperatures of formation of the diagenetic clays, based on K/Ar ages and burial history curves, are 66-125 degrees C for the IMII illite/smectite and 140-195 degrees C for the illite. These data suggest the existence of high geothermal gradients during the Cretaceous and a diminution of the geothermal gradient from Cretaceous to present. The 18 O/ 16 O ratios indicate that diagenetic fluids were enriched in 18 O with delta 18 O values between +1.7 and +9.5 per rail for the IMII illite/smectite and between +5.6 and +10.5 per mil for the K-feldspar, dolomite and illite cements. Such values preclude any significant, direct contribution of meteoric water. These 18 O-rich brines probably derived from the Aptian evaporites overlying the sediments studied.

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