Pre-rift Phase Research Articles

Depositional environments of the Early Proterozoic Espanola Formation, Ontario, Canada

The carbonate-rich Espanola Formation forms part of the Huronian Supergroup, deposited in Early Proterozoic time between about 2.5 and 2.1 Ga ago. The Espanola Formation overlies glacigenic diamictites of the Bruce Formation and is gradationally overlain by fluvial sandstones of the Serpent Formation. In the southern part of the outcrop belt, the Espanola Formation comprises a lower limestone member, a middle siltstone member, and an upper heterolithic member. These rocks record what may be the first marine incursion in the early Huronian and perhaps indicate a pre-rift phase of sedimentation. The limestone and siltstone members reflect low-energy conditions with sporadic influxes of fine-grained siliciclastics and carbonate debris from turbidity or storm-derived currents. Deposition took place subtidally, either in a shallow-marine setting or in a large lake, following the end of Bruce glaciation. Deposition of the coarser grained heterolithic member took place in higher energy environments, dominated by shallow-marine tide and storm processes.In the study area, most carbonate is detrital in origin. Paleocurrents suggest a northerly source. The restricted nature of the basin, postglacial warming, and shallower marine conditions could have been factors in carbonate precipitation.

Structural Framework, Stratigraphy, and Evolution of Brazilian Marginal Basins

The structural framework of the Brazilian continental margin is basically composed of eight structural types: antithetic tilted step-fault blocks, synthetic untilted step-fault blocks, structural inversion axes, hinges with compensation grabens, homoclinal structures, growth faults with rollovers, diapirs, and igneous structures. The antithetic titled and synthetic untilted step-fault blocks are considered as synchronous, complementary structural systems, separated by an inversion axis. They originated during the rift phase of the South American and African plates; genesis of the hinge and the homocline is related to the divergent migration of those plates. Two evaporitic cycles (Paripueira and Ibura) were differentiated in the Sergipe-Alagoas type basin and tentatively correlated to the evaporitic section of other Brazilian marginal basins. It is assumed that the Paripueira evaporites could have been continuous with their African correlatives, but the Ibura evaporites are marginal deposits and were never continuous with their west African equivalents. Four phases are considered in the evolution of the Brazilian marginal basins: pre-rift, rift, transitional, and drift. During the pre-rift phase (Late Jurassic-Early Cretaceous), continental sediments were deposited in peripheral intracratonic basins. In the rift phase (Early Cretaceous), the breakup of the continental crust of the Gondwana continent gave rise to a central graben and rift valleys where lacustrine sediments were deposited. The transitional phase (Aptian) developed under relative tectonic stability, when evaporitic and clastic lacustrine sequences were being deposited. In the drift phase (Albian to Holocene), a regional homoclinal structure developed, consisting of two distinct sedimentary sequences, a lower clastic-carbonate and an upper clastic. From the Albian to the Holocene Epoch, structures associated to plastic displacement of salt or shale developed in many Brazilian marginal basins. The diapirism deformed mainly Upper Cretaceous strata in proximal areas, and younger strata in distal areas lying, in some places, on the ocean floor. Two phases of major igneous activity occurred: one in the Early Cretaceous associated with the rift phase of the Gondwana continent, and the other in the Tertiary (Oligocene-Miocene) during the migration phase of the South American and African plates.