Episodes of Source‐sediment Deposition (1)†

The very unequal distribution of prolific oil basins, in both space and time, is due less to capricious preservation of the oil than to the relative rarity of rich, effective source sediments. There appear to have been only four intervals during which effective source sediments achieved geographically widespread deposition: Frasnian‐Famennian (360‐340 m.y), Westphalian‐Kungurian (410‐250 m.y), Kimmeridgian‐Campanian (150‐70 m.y), and Oligocene‐Middle Miocene (35‐12 m.y), Association with major orogenies (plate reorganizations) involves short but important time lags. Each reorganization involved the creation of a latitudinal (east‐west) ocean, flanked on north and south by plates carrying continents which were themselves discontinuously surrounded by new mountains (some of which constituted microplates). The latitudinal ocean first widened, causing major transgression of the continental platforms over either regional unconformities or the tops of Flysch sequences. The site of plate juncture then shifted, leading to partial re‐closure of the ocean and a second orogenic episode of either Andean or collision type. No new source sediments are associated with this second, terminating orogeny except in extensional basins remote from its effects. Thus, source sediments are phenomena of an inter‐orogenic but internally turbulent episode, coincident with a cycle of regression‐transgression‐regression. The sediments are direct analogues of the oceanic “Horizon B” deposited on continental crust. As the present Earth contains no latitudinal ocean flanked by partly mountainous continental margins, the framework cannot be properly described in present terms. Source‐sediment basins were not associated with the kinds of plate boundaries seen at present, but with margins for which initial passive extension was either aborted before the oceanic stage was reached, or was reversed and the margins converted to “pre‐Andean” Andean types having their offshore arcs under compression and not under extension. It is because both boundaries of earlier latitudinal oceans lie now in the northern hemisphere that nearly all oil is in the northern hemisphere. Because the collision orogeny between Eurasia and Gondwana during the late Paleozoic produced a severe metamorphic, platform‐creating event, and its time‐equivalent in North America did not, it is only in North America that the oil source sediment associated with that orogeny (those of our second interval) survived to become effective. The Mississippian‐to‐Jurassic interval is otherwise gas‐prone on a worldwide basis. A post‐hercynian orogeny, during the Triassic, is best developed in the Gondwana continents and their satellite microplate fragments, and may have associated source sediments that have been overlooked. Because the late Mesozoic closure of Tethys progressed from west to east, and did not extend beyond the present Persian Gulf until the Tertiary, none of the important Jura‐Cretaceous source sediments associated with it extend east of the Persian Gulf. Effective source sediment deposition world over ended at the time of the Middle Miocene orogenic episode (18‐10 m.y.). There is no significant oil production associated with basins formed since the Middle Miocene. Oil source sediments are not normally associated with the margins of wholly oceanic plates (Pacific. Nazca. Kula). now or in the past: the rich Californian basins are a unique exception made possible by fortuitous boundary conditions. The source sediment maxima appear to correlate directly with overall sedimentation rates and with minimum depths of the oxygen‐deficient layer. The maxima display ambiguous correlation with climate, ocean temperatures, depths to the CCD (though shallowing of that level appears to be favourable), and faunal diversification. cosmopolitanism. or extinction. They exhibit negative correlation with marine phosphorites and evaporites—introducing the possibility that these are the analogues of oil‐source sediments under different tectonic and climatic conditions—and with plume discharges from the mantle. They exhibit no correlation at all with seafloor spreading rates or with magnetic reversals.