Rollover anticlines

Abstract: The structure of the Papuan fold belt differs substantially from the classical thin-skinned structures of the Canadian Rockies, mainly due to the weak and broken Papuan lithosphere and incompetent clastic section, compared to the strong cratonic lithosphere in Canada overlain by limestones and quartzites. The Papuan margin was dissected by Mesozoic extensional faults, in part reactivated during Neogene compression, which created basement-involved anticlines or elevated plateaus in the Papuan fold belt. The sedimentary section in the fold belt is deformed into large ramp anticlines in the southwest, smaller Miocene and Mesozoic duplexes in the center, and fault-propagation folds in the northeast. Hydrocarbon discoveries to date have occurred in the southwestern anticlines, which are interpreted as reactivated extensional structures. The precursor rollover anticlines trapped oil generated in the Late Cretaceous such that present accumulations probably result from preservation of that oil during Neogene thrusting. Mesozoic duplex structures are likely to be prospective but difficult to define, and fault propagation folds rely upon unproven Upper Cretaceous sandstones as a reservoir.

Abstract: Analysis of multichannel seismic reflection profiles reveals that the northern East China Sea shelf basin experienced two phases of rifting, followed by regional subsidence. The initial rifting in the Late Cretaceous created a series of grabens and half grabens, filled by alluvial and fluviolacustrine deposits. Regional uplift and folding (Yuquan movement) in the late Eocene–early Oligocene terminated the initial rifting. Rifting resumed in the early Oligocene, while alluvial and fluviolacustrine deposition continued to prevail. A second phase of uplift in the early Miocene terminated the rifting, marking the transition to the postrift phase. The early postrift phase (early Miocene–late Miocene) is characterized by regional subsidence and westward and northwestward marine transgression. Inversion (Longjing movement) in the late Miocene interrupted the postrift subsidence, resulting in an extensive thrust-fold belt in the eastern part of the area. The entire area entered a stage of regional subsidence again and has become a broad continental shelf. Source rocks include synrift lacustrine facies, fluvial shales, and coal beds. Synrift fluvial, lacustrine, and deltaic deposits, postrift littoral and/or shallow-marine sandstones, and fractured basement have the potential to provide reservoirs. Various types of hydrocarbon traps (e.g., faulted anticlines, overthrusts, rollover anticlines, faults, unconformity traps, combination structural-stratigraphic traps, weathered basement, and stratigraphic traps) are recognized, but many of these traps have not been tested.

Abstract: ¶The crustal structure of the Campania continental margin is synthesized from outcrop, seismic reflection and gravimetric data. Outcrop and subsurface geological data reveal the presence of NE–SW faults, E–W faults and NW–SE faults. An older extensional event occurred along NW–SE faults and was followed by the main extensional event linked to the activity of NE–SW normal faults. The latter were active between 700 and 400 ka producing half-grabens filled by more than 5 km of Quaternary deposits. The stratigraphic signature of these tectonic events corresponds to a Lower Pleistocene marine unconformity-bounded unit overlain by Middle Pleistocene rocks belonging to a transgressive-regressive cycle. A crustal section of the Campania margin displays an asymmetric linked fault system characterized by a 10–12 km-deep main detachment level, listric normal faults and rollover anticlines. Structural and stratigraphic data document that the inception of volcanic activity at Vesuvius occurred at 400 ka, just after the main extensional event, and the volcano is located at the margin of a rollover anticline.