Paleoambiente do Grupo Serra Grande, borda Leste da Bacia do Parnaíba, localidade de Ipueiras, Estado do Ceará

Abstract

The transition between the Ordovician and Silurian periods in the western portion of the Gondwana supercontinent was marked by a long glacial period, which had its peak in the Hirnantian (~ 445 Ma). This event occurred during a long greenhouse climatic period, in which the CO2 content in the atmosphere was five times greater than the current one. The supercontinent Gondwana migration towards the South Pole was concomitant with astronomical factors such as changes in the rotation of the earth and decreased solar radiation that favoured the ice-sheets growth. The end of the glaciation was marked by the most substantial increase in the eustatic sea-level recorded in the earth history, generating significant marine transgressions, which started at Llandovery (~ 443 Ma) and ended at Ludlow (~ 423 Ma). One of the best records of these events within the West Gondwana context is the Serra Grande Group, which represents an Ordovician-Devonian siliciclastic succession present in the Parnaíba Basin, northeastern Brazil. This sequence outcropping on the eastern edge of the basin is divided into three formations: Ipu, Tianguá, and Jaicós. Previously work of this Group has a strictly regional and lithostratigraphic character, which has not allowed determining the depositional environments and systems based on a precise stratigraphic framework. This work presents new interpretations of depositional systems present in the Serra Grande Group record, based on sedimentological and stratigraphic analyses of the outcrops located in the Ipueiras region, Ceará State, Northeast Brazil. The detailed faciological description of this siliciclastic succession had as main objective to propose a depositional and evolutionary model for these deposits. Outcrop-based facies and stratigraphical analysis of this siliciclastic succession indicated fluvial, glacial, and coastal depositional systems grouped on seven facies associations (FA). Deposits of sheet braided plains (AF1), consisting of coarse and conglomerate sandstones with cross-stratification. Glacial deposits (AF2 and AF3) corresponding to massive conglomerate, thick sandstone with cross-stratification and massive diamictites. Stratified diamictite with drop clasts were deposited in a glacial-marine environment (AF4). Laminated black shales (AF5) are interpreted as deposited in offshore settings during post-glacial stages. Deltaic deposits (AF6) consist of laminated siltite and sandstone with sigmoidal cross-stratification and occurrences of the ichnogenus Arthrophycus. This fluvial-glacial-deltaic succession is erosively overlaid by sandstones with trough cross-stratification, deposited within a channelled braided plain (AF7). This proposal confirms partially the previous paleoenvironmental interpretation discarding the presence of subaerial alluvial fan and outwash plain deposits for this unit. The sequence stratigraphy of Serra Grande Group was refined using mainly the coherent interpretation of the key surfaces and comparing the system tracts with the global sea-level curve providing a more robust third-order sequential evolutive model that includes three depositional sequences. The development of an extensive fluvial system (AF1, Ipu Formation) directly over the basin basement, with thicknesses of hundreds of meters, suggests a perennial river probably supplied by mountainous regions to the west of Gondwana. These deposits of middle Ordovician age were generated under low sea conditions belonging to sequence 1, and are truncated by a significant unconformity that possibly removed the transgressive and highstand strata. This unconformity produced by the glacial dynamics removed approximately 25 Ma from the sedimentary record of the basin. The second sequence started with the advance of the ice-sheets in the early Silurian, with the installation of an ice-contact fan (AF2 and AF3, Ipu Formation), in lowstand conditions. During the icesheet retreat, large amounts of water and debris were released, allowing the deposition of stratified diamictites with the presence of dropstones (AF4, Ipu Formation). The transgressive phase culminated in an increase in sea level and subsequent deposition of black shales from AF5 (Tianguá Formation), during the middle to late Silurian period. The highstand phase in the late Silurian is marked by the installation of a delta system (AF6, Tianguá Formation). In the early Devonian, an expressive new stage of sea-level fall produced an unconformity on the continental shelf concomitant with the progradation of a braided river system (AF7, Jaicós Formation), under lowstand phase of sequence 3. The Serra Grande Group paleoenvironmental and paleoclimatic reconstruction provided important sedimentological and stratigraphic elements for understanding the pre- and post-glacial transgressive and regressive phases, as well as assisting in the paleogeographic reconstruction of West Gondwana ice-sheets during the Silurian.