Basin Research - Volume 2, Issue 4, 1989

Abstract The uniform stretching model has been applied to seismic reflection profiles and well‐log information from the Pearl River Mouth Basin on the northern flank of the South China Sea. Stretching factors were calculated from subsidence curves determined from the stratigraphy by using the backstripping technique to remove the effects of compaction and sediment loading. Variations in rift topography, palaeobathymetry and global sea‐level v/ere taken into account. We argue that the Pearl River Mouth Basin formed by lithospheric extension by a factor of about 1.8, lasting from Late Cretaceous to late Oligocene times. Stretching factors calculated from subsidence agree with those determined from the geometry of normal faulting and from crustal thinning. Thus there is no indication of a significant discrepancy between the different estimates of stretching. The geometry of faulting suggests that considerable amounts of local footwall uplift occurred during the rifting period. Small differences between the observed and calculated subsidence curves (? 400 m in the middle Miocene) are best explained by minor amounts of extension (β? 1.1). The time‐temperature history of sediments within the basin has also been calculated so that expected vitrinite reflectance and oil abundance could be determined. The results are consistent with each other and are in reasonable agreement with observations from wells.

Abstract Using a detailed petrographical procedure conceived for arenites rich in carbonate clasts, the influence of tectonism and eustacy on silicate/carbonate cycles of the Eocene Hecho Turbidite Complex has been tested, and the palaeogeography of the source/basin system outlined.

Both extrabasinal and intrabasinal sources of sediments were active during basin filling. The extrabasinal source terrains, located in the southern sector of the basin, were made of the Pyrenean crystalline basement (granites, gneisses and phyllites) overlain mainly by carbonate rocks (Cretaceous limestones and dolostones, minor chert and siltstones). The intrabasinal sources, represented by foramol shelf carbonate factories, provided penecontemporaneous carbonate bioclasts, intraclasts and peloidal grains.

Foreland thrusting in the South‐Central Pyrenees has acted as the major control on the composition and architecture of the Hecho Turbidite Complex. Strong uplift of old silicate and carbonate source terrains during southward thrust propagation was responsible for erosion, swamping and/or reduction of shelfal areas, and gave rise to siliciclastic and carbonate basinal sequences (silicate arenites and calclithites) during lowstand stages. Conversely, hybrid arenites (mixture of extrabasinal and intrabasinal grains) originated from resedimentation of marginal shelf sediments produced in carbonate factories active during the initial phase of sea‐level rise. Hybrid arenites with minor intrabasinal content also formed during one stage of relative sea‐level fall from the erosion of previously accumulated highstand complexes.

During resedimentation processes, hybrid sands underwent marked hydraulic selection documented by deposits depleted in carbonate grains in the channel area, and by thin‐bedded turbidites rich in platy‐skeletal fragments, low‐density peloids and void‐rich bioclasts down‐basin.

Abstract The Devono‐Carboniferous Horton Group of Cape Breton Island was mostly deposited in two fault‐bounded asymmetric sub‐basins which were part of a large intracontinental rift system. This system lay at a palaeolatitude of about 10–15^o S–a warm, semi‐arid climate. The half‐graben sub‐basins had opposed polarity, were approximately 100 times 50 km in size and were separated by a narrow zone of elevated Acadian basement. These features are common to the adjacent structural segments of known rifts, and are unlike those of transtensive pull‐apart systems. Sedimentation occurred in four successive depositional systems which reflect a tectonic evolution of increased and then decreased extensional subsidence through the 8–12 Myr interval represented. Post‐Acadian sedimentation began with System 1 bimodal volcanics and grey distal braided fluvial sediments deposited in a slowly subsiding broad linear sag basin. System 2 consists of reddened braidplain sediments near fault‐bounded margins and mudflat/playa sediments in sub‐basin centres, deposited in two discrete asymmetric sub‐basins with a general upward‐fining trend. Gradual expansion of the mudflat setting and confinement of coarse marginal fades is interpreted as a response to increasingly rapid and deep fault‐bounded subsidence. Depositional System 3, is a complex of grey lacustrine offshore, shoreline and fan delta facies deposited in two adjacent half‐graben segments with opposed polarity of asymmetry. An increased rate of tectonic subsidence allowed a large standing body of water to accumulate lacustrine sediments along the axis of each sub‐basin during this phase of maximum subsidence. System 4 consists of reddened proximal alluvial fan, medial fluvial and distal grey meandering fluvial/floodplain sediments which accumulated in sub‐basins with fault‐bounded margins and asymmetry identical to those of earlier systems, indicating a continuation of tectonic style. However, an overall coarsening‐upward trend indicates waning of active fault‐related subsidence and consequent progradation of marginal coarse wedges to fill the sub‐basins. Rapid marine transgression and deposition of Windsor Group carbonates, evaporites and elastics continued within a more extensive rift basin during renewed fault‐bounded subsidence.