Interações vulcano-sedimentares no cretáceo superior da bacia sanfranciscana, mesorregião do alto paranaíba, minas gerais.

Abstract

The study of dynamic interactions between magmas, lavas, and pyroclastic deposits of different geochemical affinities with nonconsolidated or poorly consolidated sediments provides important information regarding paleoenvironmental and paleovolcanological conditions. Such interactions are more common in marine and lacustrine successions but can also develop in continental subaerial environments and are directly related to phreatomagmatic explosions and lava and pyroclastic flows. In the region between Estrela do Sul and Cascalho Rico, Alto Panaraíba Mesoregion, Minas Gerais State, aeolian deposits of the Posse Formation (Urucuia Group) occur interbedded and are intruded and covered by lavas and pyroclastic deposits of the Patos Formation (Mata da Corda Group) from the Upper Cretaceous of the Sanfranciscana Basin. Close to the contact between tuff-breccias and lapilli-tuffs with quartz-sandstones, dispersed peperites were observed, characterized by a mixture of amoeboid, globular and block (jigsawfit) juvenile clasts immersed in a fine- to medium-grained sandy matrix. Peperites were initially formed by the ballistic fall of pyroclastic fragments (lapilli and bomb) within nonconsolidated sandy sediments of liquified or hydroplastic character. The nonconsolidated state and the presence of water in the aeolian system are evidenced by a series of soft-sediment deformation structures, including clastic dikes, convolute laminations, synsedimentary faults, water escape structures and intraformational breccias. Mixing between aeolian sediments and pyroclastic fragments was mainly due to the density differences between the hot juvenile low viscosity clasts and humid host sediments along a preferred horizon. Instantly after the ballistic fall, it is suggested that the immersion of the pyroclasts gave rise to juvenile clasts with fluidal morphologies (amoeboid and globular), suggesting interaction with sediments still in a ductile regime and with low viscosity and relatively hot. The change in morphology from juvenile clasts with amoeboid forms to microfractured block forms is interpreted as a combination of a decrease in the temperature of juvenile clasts and an increase in viscosity that led to brittle behavior of the clasts. After the formation of the peperites, the aeolian system remained active until it was eventually covered by thick lava flows.