Abstract
ABSTRACTGlaciotectonic features studied in the siliciclastic deposits of Cabeças Formation, Upper Devonian, represent the first evidence of Famennian glaciation in Southeastern Parnaíba Basin, Brazil. Outcrop-based stratigraphic and facies analyses combined with geometric-structural studies of these deposits allowed defining three facies association (FA). They represent the advance-retreat cycle of a glacier. There are: delta front facies association (FA1) composed of massive mudstone, sigmoidal, medium-grained sandstone with cross-bedding and massive conglomerate organized in coarsening- and thickening-upward cycles; subglacial facies association (FA2) with massive, pebbly diamictite (sandstone, mudstone and volcanic pebbles) and deformational features, such as intraformational breccia, clastic dikes and sills of diamictite, folds, thrust and normal faults, sandstone pods and detachment surface; and melt-out delta front facies associations (FA3), which include massive or bedded (sigmoidal cross-bedding or parallel bedding) sandstones. Three depositional phases can be indicated to Cabeças Formation: installation of a delta system (FA1) supplied by uplifted areas in the Southeastern border of the basin; coastal glacier advance causing tangential substrate shearing and erosion (FA1) in the subglacial zone (FA2), thus developing detachment surface, disruption and rotation of sand beds or pods immersed in a diamicton; and retreat of glaciers accompanied by relative sea level-rise, installation of a high-energy melt-out delta (FA3) and unloading due to ice retreat that generates normal faults, mass landslide, folding and injection dykes and sills. The continuous sea-level rise led to the deposition of fine-grained strata of Longá Formation in the offshore/shoreface transition in the Early Carboniferous.
Highlights
Gondwana glacigenic successions have been the focus of several recent stratigraphic studies, mainly due to their potential as possible petroleum systems
One of the key reasons is the favorable condition for hydrocarbon generation that is associated with the presence of thick layers of transgressive post-glacial black shale, whereas sandstones, sandwiched by fine-grained glacio-marine facies, represent possible reservoir rocks (Vesely et al 2007, Assine & Vesely 2008, Fielding et al 2012)
The glacial deposits in Western Gondwana are well preserved in the Paleozoic basins of South America (Cunha et al 1994, Loboziak et al 2000, Streel et al 2000, Isaacson et al 2008)
Summary
Gondwana glacigenic successions have been the focus of several recent stratigraphic studies, mainly due to their potential as possible petroleum systems. Moore (1963) and Caputo (1985) analyzed outcrops of Cabeças Formation in the southwestern border of Parnaíba Basin and described foliated tillites with striated and faceted pebbles between layers of sandstones with massive bedding and waterscape structures. These investigators concluded that the glacier movement plastically deformed the sediments of Cabeças Formation, not affecting the older and younger Longá and Pimenteiras formations, respectively. Grahn et al (2006) and Streel et al (2013) inferred a Late Famennian age for these tillites based on the presence of miospores of LE (R. lepidophyta – H. explanatus) and LN (R. lepidophyta – V. nitidus) zones established to Western Europe
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