versión impresa ISSN 0001-3765
An. Acad. Bras. Ciênc. v.72 n.4 Rio de Janeiro dic. 2000
DIAMICTITE FACIES OF THE ITARARÉ SUBGROUP (LATE PALEOZOIC), IN THE SOUTHERN PARANÁ BASIN, BRAZIL*
JOSÉ ROBERTO CANUTO, ANTONIO C. ROCHA-CAMPOS and PAULO R. SANTOS
Instituto de Geociências, USP, São Paulo, SP.
Diagnostic features of glacigenic deposits, both in the continental and marine environments, have been extensively discussed in the last decades.
A special facies classification of these rocks similar to that used by Canuto (1993) with some improvements, is here proposed, with practical approaches to field applications, based on lithology, texture, structure, clast orientation, color, geometry, lateral extension and, in some cases, presence of ichnofossils, microfossils and palinologic content.
Compact massive diamictite facies. This facies is composed by compacted massive diamictite, with shearing at the base. It occurs in the form of discontinuous bodies plastered in irregularities of the substratum, whose surface is flat, polished and estriated, with eventual "crescentic marks''. The substratum, close to the contact, also reveals to have been sheared. It corresponds to the lodgement tillite. Non-compact, massive diamictite facies. It is a massive tillite, formed from the slow melting of stagnated ice masses. Tabular diamictite facies. It is composed of tabular, massive diamictite, formed by cohesive debris flows, having folded and faulted sandstone layers, which may be sheared, and disrupted, sometimes forming "sandstone balls'', with clasts generaly without orientation. The lower contact is sharp, and the upper can be transitional to fine clastics. Lenticular diamictite facies. This facies is formed by channelized massive diamictite, deposited by cohesive debris flows. Its characteristics are similar to those of the tabular diamictite facies, differing by its geometry amalgamated and stacked channels, associated with the movement of lenticular channelized lobes tens of meters thick as a whole. The lower contact is sharp, and the upper can be transitional to fine clastics. Clasts may be oriented parallel to the bedding. Deformed interbedded sandstone and diamictite facies. It occurs as extensive tabular bodies, with metric thickness, of intercalated deformed diamictite and sandstone, with beds showing slump folds and faults, sometimes forming a chaotic mixture. It is interpreted as formed by gravitational flows of glaciogenic sediments, deposited at the margins of a water body, perhaps marine, and covered by shoreface sandstones. Laminated clast rich diamictite facies. Laminae are piled up with obscure contacts, or with suspension deposits. Bodies are up to tens of meters thick and were deposited by dense flows, intermediate between debris flows and turbidites. This facies differs from the debris flows by being turbulent flows which erode the substrate. Laminated diamictite with lenses of massive diamictite facies. Lamination is given by subparallels discontinuities or joints, without associated textural variation, eventually showing clast orientation. Lamination is undulate and includes lenticular, elongated bodies of massive diamictite. The diamictite is interpreted as formed by gravitational sedimentary flows. (December 10, 1999)
*Supported by FAPESP, Proc. 13973-2.