Late paleozoic glacial boulder pavements from Jumirim, SP (Itararé subgroup): new evidence on origin

LATE PALEOZOIC GLACIAL BOULDER PAVEMENTS FROM JUMIRIM, SP (ITARARÉ SUBGROUP): NEW EVIDENCE ON ORIGIN^*

M

AURICIO P. LUCIO and ANTONIO C. ROCHA-CAMPOS

Instituto de Geociências, USP, São Paulo, SP.

Mapping and facies analysis of diamictites containing boulder pavements and associated rocks from the upper part of the Itararé Subgroup in Jumirim, SP, furnished additional evidence on the glacial paleoenvironmental context and origin of the clast concentrations.

Five facies associations have been mapped and interpreted in terms of their paleoenvironment. Associations I and II were formed mainly by the settling of muds from suspended sediments plumes derived from subglacial meltwater plus a variable contribution of debris from floating ice. Evidence of reworking of glaciogenic debris by bottom traction currents is also present; Association III represents a subaqueous clastic fan fed by meltwater currents; Association IV is a fluvio-deltaic subaqueous (or partly subaerial) outwash wedge that passes distally to a submarine fan associated with flows of glaciogenic debris; Association V includes a lower subglacial diamictite (lodgement tillite) resting on a glaciotectonized substratum, overlain by a possible meltout diamictite (meltout tillite).

Boulder pavements A and B occur respectively in the lower and upper diamictites of Association V. Environment of deposition of the lower diamictite implies in the formation of the boulder pavement during deposition of a subglacial till. Formation of the pavement may thus have been through processes related to lodgement. Interpretation of the upper diamictite as a subglacial meltout tillite and origin of boulder pavement B in this context are less clear. It is preliminary interpreted as a structure inherited from the basal transport zone of ice.

Rocks studied record the permanence of glacial influence during deposition of the uppermost beds of the Itararé Subgroup in northern Paraná Basin. The general environmental setting is interpreted as a subaqueous (possibly marine) basin marginally affected by a late advance and retreat of the Kaokoveld glacial lobe. (December 10, 1999)

^*

Supported by FAPESP, Proc. 91/0546.2.

Publication Dates

• Publication in this collection
  05 Jan 2001
• Date of issue
  Dec 2000