Post-rift Cretaceous siliciclastic rocks of early Valanginian to early Aptian age in the Pletmos Basin, southern offshore, South Africa, display well-developed, seismically resolvable, type 1, fourth-order (simple) depositional sequences arranged in progradational and aggradational sequence sets. Each set is a depositional systems tract within third-order composite sequences. The fourth-order stacking patterns displayed by the sequence sets primarily reflect the strong influence of third-order eustatic cycles superimposed on tectonic subsidence. Tectonic subsidence episodes and their terminal uplift events are documented by the stacking patterns of component sequence sets and the tectonically enhanced erosional boundaries of respective second-order supersequences. Conjunc ion of second-, third-, and even fourth-order falling inflection points produces maximum subaerial erosion and most favorable lowstand reservoir sandstone potential.

Following uplift and second-order erosion of the drift onset unconformity (126 Ma), initial rapid subsidence was followed by a gradual decrease in subsidence (accommodation) rates from the early Valanginian (126 Ma) to latest Hauterivian (117.5 Ma). This first post-rift tectonic episode occurred in response to inferred diminishing transtensional (rift) stress and coincided with the deposition of 17 fourth-order sequences arranged in sequence sets within third-order composite sequences. The sets alternately comprise three or five individual simple sequences averaging 0.33 and 0.12 m.y. in duration,

End_Page 43-------------------------

respectively. The sets display stacking patterns grading (upward) from fully aggradational to progradational. This first post-rift supersequence (10-m.y. duration) was terminated by uplift and intensive second-order erosion at 117.5 Ma.

From earliest Barremian (117.5 Ma) to early Aptian (112 Ma) time, initial accelerated subsidence was followed by decreasing subsidence (accommodation) rates. This second post-rift tectonic episode was probably produced by transpressional(?) stress generated by right-lateral movement of the Falkland Plateau past the southern margin of the African plate. During the 4 m.y. from 116 to 112 Ma, 33 fourth-order simple sequences were deposited, each with an average cycle (Milankovitch?) duration of 0.12 m.y. High-frequency sequences are stacked in alternating progradational and aggradational sets, and the sets are, respectively, lowstand and highstand systems tracts within three third-order composite sequences. This second post-rift supersequence (5.5-m.y. duration) was also terminated by up ift and intensive second-order erosion at 112 Ma.

Deposition of the fourth-order sequences and sequence sets was probably in response to relative fourth-order sea level cycles produced by (1) decreasing space-added accommodation (subsidence) rates, (2) high sediment-supply rates (~1.8 m/k.y.), and (3) fourth-order eustatic cycles superposed on the falling and rising limbs of third-order eustatic cycles. These two initial post-rift, second-order, transtensional and subsequent transpressional(?) tectonic episodes were followed by release of external stress, permitting thermal-decay subsidence rates and deposition of principally third-order sequences during the remaining Cretaceous supercycles.