Origin and preservation of stratigraphically repeated, glacially striated surfaces in the Itararé Subgroup (Late Paleozoic) in Palmeira, State of Paraná

ORIGIN AND PRESERVATION OF STRATIGRAPHICALLY REPEATED, GLACIALLY STRIATED SURFACES IN THE ITARARÉ SUBGROUP (LATE PALEOZOIC) IN PALMEIRA, STATE OF PARANÁ ^*

IVO TROSDTORF JUNIOR, ANTONIO C. ROCHA-CAMPOS, ALEXANDRE TOMIO, PAULO R. SANTOS AND JOSÉ R. CANUTO

Instituto de Geociências, USP, São Paulo, SP.

Rocha-Campos et. al. (1999, Anais ABC, 71: 841) interpreted the multiple striated surfaces from the Itararé Subgroup in Palmeira as having been abraded subglacially on soft sediment by a terrestrial glacier with an oscillating margin. Four to five surfaces are exposed, cut on top of a fluvio-glacial, fine-medium, cross-bedded sandstone.

Important characteristics of the surfaces are: absence of deformation of the sandstone; thin sand beds (dm) separating surfaces; occurrence of striated areas side by side with non-striated and ripple-marked areas; the probable fluvio-glacial nature of the sandstone; absence of diamictite in the section; absence of lodged clasts; and presence of centimetric beds/laminae of siltstone on top of most of the surfaces and a decimetric bed of siltstone on top of the uppermost surface.

These features indicate a possible combination of low effective pressure of the glacier, a rigid substrate, sandy glacier bed with low pore water pressure and possible presence of a subglacial water layer. Glacial abrasion occurred by ploughing by ice protuberances under the glacier that was not completely in contact with its bed, probably partially floating on a thin water layer.

Still controversial is why the surfaces have been preserved. Evidence of subglacial carbonate cementation of the surfaces was not found and preservation of the subglacial features by cold-based deglaciation can be ruled out by the absence of associated permafrost features.

On the other hand presence of the silt layers above the surfaces suggests that they may have been protected from further erosion by deposition of a cover of fine sediment on top of them. This raises the question on whether the sliding glacier was subaquatically grounded, or if terrestrially based, the abraded surfaces were immediately covered by water of ephemeral glacial lakes or pools after retreat of the ice. The second explanation seems more plausible to us in view of the fluvio-glacial nature of the striated sandstone. Another vexing question refers to the absence of subglacial till deposits in the section. We propose that movement of the Paraná lobe on the underlying Furnas sandstone led to entrainment of large amounts of sand in the basal debris layer of the glacier, later released and reworked by fluvio-glacial action. — ( December 8, 2000 )

Publication Dates

• Publication in this collection
  08 Oct 2001
• Date of issue
  Sept 2001