RELATIONSHIP BETWEEN AGES OF SHIELDS, DIPS OF ADJACENT SEDIMENTARY BASEMENTS, AND OCCURRENCE OF OIL AND GAS

Abstract

Crystalline shields can become stable at different times. This stability is commonly marked by the end of magmatism and compressive deformation and the beginning of deposition of platformal sedimentary sequences. The margins of the shields are then the basements for undeformed sediments that dip away from the exposed crystalline rocks.Dips of basements on shield margins directly underlying Phanerozoic sediments have been determined in seven areas in which the margin has not been altered by rifting or other tectonic activity. The adjacent shields have stabilization ages ranging from more than 3,000 m.y. to 600 m.y. The basement dips (tilts) increase progressively from about 3 m/km for older shields to 16 m/km adjacent to the youngest shield (the Nubian‐Arabian shield). These tilts can only approximately be recalculated into uplift rates of the shields or subsidence rates of their margins. The dips are, however, consistent with Phanerozoic uplift rates of Archean cratons of less than 5 m/m.y., or comparable subsidence rates of the margins; such low uplift rates could be caused almost wholly by erosion and isostatic adjustment. Younger shields have undergone some other, as yet unidentified, process that causes more rapid uplift.The higher dips of basements around younger shields apparently promote the development of thick sections of mildly‐deformed Phanerozoic sediments. Sequences of this type are commonly favorable for accumulation of commercial concentrations of oil and gas. A correlation has been found between age of shield, basement dip, and hydrocarbon occurrence around the western edge of the Canadian shield and the northern edge of Africa. The data show that oil and gas are more abundant in Phanerozoic sediments adjacent to younger shields, which supports the common observation that the world's largest hydrocarbon accumulation (the Arabian Gulf) is adjacent to the world's youngest shield (the Nubian‐Arabian shield).This criterion for hydrocarbon accumulation may be of some value in exploration of little‐known areas. For example, basements of different ages occur adjacent to the overthrust belt of the northern Rockies, and the generally‐young age of Precambrian basement in the eastern US may indicate considerable thickness of sediment under crystalline thrusts in the Appalachians. Different ages of shields in different parts of China may also have broadly affected sedimentary patterns in that country.