Register of increasing continentalization and palaeoenvironmental changes in the west-central pangaea during the Permian-Triassic, Parnaiba Basin, Northern Brazil

摘要

The increasing desertification in the continental interior of Pangaea during the Permian-Triassic transition was of fundamental importance in the Earth landscape and life evolution. In the west-central Pangaea, northern Brazil, acid-saline-lake systems were gradually succeeded by dry aeolian ergs due to intense continentalization in arid climate setting. Detailed facies and architectural analysis were applied in the Permian-Triassic succession of the Parnaiba Basin, and seven facies associations were recognized: (1) ephemeral lake, represented by greenish and reddish gray laminated mudstones interbedded with chert-rich fine-grained sandstones, (2) marginal dune field consisting of fine-to medium-grained sandstones with planar cross-bedding sets, (3) shallow perennial saline lake consisting dominantly of red laminated mudstones with discontinuous layers of sigmoidal sandstones, (4) playa lake/thy mudflat represented by reddish laminated mudstones interbedded with limestone, marl and gypsum lenses, (5) sand sheet consisting of laterally continuous beds of fine-to medium-grained sandstones with convoluted lamination, synsedimentary faults/microfaults and adhesion structures, (6) dune field formed by fine-to medium-grained sandstones with large-scale cross-bedding sets, and (7) volcanic plain, consisting of basalts interbedded with aeolian sandstones. During middle Permian, the shallow to deep ephemeral lakes occurred in large plains in the tropical zone of western to central Pangaea. The cyclicity of wet and thy lacustrine phases was triggered by changes in the groundwater level, low subsidence rate, and low accommodation space. The prolonged dry stages were characterized by the advance of the marginal dune fields as well as the establishment of large desiccated areas. In the upper Permian, the continuous process of Pangaea amalgamation led to the uplift of central and equatorial regions resulting in the retreat of epicontinental seas. Thereafter, there was the appearance of large-scale closed basins and continental saline environments. The extreme aridity conditions favored the decline of these great lakes and the development of an extensive Triassic Erg. Sand sheets occurred in the marginal areas of the erg, containing abundant ephemeral ponds and dispersed aeolian dunes. Extensive dune fields advanced as a consequence of the sediment availability increase, whereas deflation surfaces were produced by the widespread output of sediments. The total interruption of sediment availability to the erg in the upper Triassic provided an extreme and regional deflation surface overlaid by eoJurassic volcanic rocks, associated with the Pangaea breakup.