Famennian glaciation in the eastern side of Parnaíba Basin, Brazil: evidence of advance and retreat of glacier in Cabeças Formation

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.摘要：本研究针对巴西帕纳伊巴盆地（Parnaíba Basin）东南部上泥盆统（Upper Devonian）卡贝萨斯组（Cabeças Formation）硅质碎屑沉积（siliciclastic deposits）中的冰川构造特征（glaciotectonic features）展开分析，该类特征为巴西帕纳伊巴盆地东南部法门期（Famennian）冰川作用提供了首个实证依据。基于露头的地层学与岩相分析，结合该沉积体的几何-构造研究，本研究共厘定出3种相组合（facies association，FA），它们完整代表了一次冰川的进退旋回：其一为三角洲前缘相组合（FA1），由块状泥岩、具S型交错层理的中粒砂岩与块状砾岩组成，构成向上变粗变厚的沉积旋回；其二为冰下相组合（FA2），发育由砂岩、泥岩及火山砾构成的块状含砾泥砾岩（diamictite），并伴生多种变形构造，包括层内角砾岩、泥砾岩碎屑岩墙与岩床、褶皱、逆冲及正断层、砂岩透镜体与滑脱面（detachment surface）；其三为融出三角洲前缘相组合（FA3），包含块状或具层理（S型交错层理或平行层理（parallel bedding））的砂岩。卡贝萨斯组可划分为3个沉积阶段：第一阶段为三角洲体系发育期（对应FA1），物源来自盆地东南部的抬升剥蚀区；第二阶段为海岸冰川前进期，冰川前进引发冰下带的基底切线剪切与侵蚀作用（对应FA1），进而形成滑脱面、泥砾岩包裹的砂层或透镜体的破裂与旋转构造；第三阶段为冰川退缩期，伴随相对海平面上升，形成高能融出三角洲（对应FA3），同时冰川退缩引发的卸载作用产生正断层、大规模滑坡、褶皱及岩墙与岩床侵入构造。持续的海平面上升最终导致早石炭世（Early Carboniferous）时期，在近海/近岸过渡带沉积了隆加组（Longá Formation）的细粒地层。