Paulo Roberto dos Santos

Late Paleozoic glaciotectonic structures in northern Paraná Basin, Brazil

Abstract Fluviatile–deltaic sediments containing thin coal layers, and glaciofluvial sediments of the Itarare Subgroup (late Paleozoic) cropping out near Cerquilho, northern Parana Basin, exhibit post-depositional deformation such as recumbent and drag folds, shear fractures, faults and shear lamination. The deformation occurs in an interval of 3–4 m thick and is confined between horizontal strata. The deformed strata are directly overlain along a horizontal erosional/tectonic contact surface by a silty–sandy, massive, 1 m thick, clast-poor tillite. Deformation shows a consistent geometry indicating a predominant vergence toward the SSW having been developed under a horizontal stress oriented from NNE to SSW. The deformation is interpreted to be of glaciotectonic origin associated with an ice readvance in the area. The glacier moved toward the SSW on relatively soft, deformable sediments, depositing the subglacial till on top of them. The tillite shows parallel, upglacier-dipping shear fractures at its base and wedge-like intrusions into the underlying sandstone, probably formed during its subglacial deposition. The tillite is overlain by cyclic (braided stream) fluviatile, sandstone beds. The top of these shows deformations (small recumbent folds and reverse faults) below another clast-poor, silty–sandy tillite. This could represent another glacial readvance with subglacial deposition of till. Occurrence of glaciotectonic deformation overlain by subglacially deposited tillites in the Cerquilho area indicates glacier readvances and permanence of the glacial influence in the upper part of the Itarare Subgroup. Absence of incorporated material from the underlying deformed sediments in the tillite and the style of the deformations suggest an unfrozen, water-saturated, mostly unconsolidated substratum. These conditions may have generated instability and fast ice flow of the glacier. They also suggest that the sediment may have most likely failed in this manner under rapidly applied stresses associated with a surging behavior of the retreating glacier.

Glacially striated, soft sediment surfaces on late Paleozoic tillite at São Luiz do Purunã, PR

Striae and furrows found on the upper surfaces of three stratigraphically superposed decimetric beds of late Paleozoic lodgement tillite of the Itarare Subgroup in the northern Parana Basin were engraved by ploughing of clasts and possibly also ice protuberances at the base of the glacier, on unconsolidated to partially consolidated sediment. Associated features indicate that the rheology of the bed varied from stiff during lodgement to soft and deformable during ploughing. Poor drainage of meltwater at the glacier-bed interface may have contributed to lower the strength of sediment to deformation. The deformed interval was probably generated during a single glacial phase or advance of a glacier grounding in a marine or lacustrine water body. Changes in the dynamics of the glacier involving slow and fast flow were correlated respectively with alternation of deposition and erosion. The proposed model is analogous to that of lodgement till complexes from the Pleistocene of the northern hemisphere. Retreat of the glacier was probably fast, followed by settling of muds on top of the upper striated and furrowed surface, and progradation of deltaic sands during post-glacial time.

Invertebrates from the Low Head Member (Polonez Cove Formation, Oligocene) at Vaureal Peak, King George Island, West Antarctica

Abstract Eight taxa of marine invertebrates, including two new bivalve species, are described from the Low Head Member of the Polonez Cove Formation (latest early Oligocene) cropping out in the Vauréal Peak area, King George Island, West Antarctica. The fossil assemblage includes representatives of Brachiopoda (genera Neothyris sp. and Liothyrella sp.), Bivalvia (Adamussium auristriatum sp. nov., ?Adamussium cf. A. alanbeui Jonkers, and Limatula (Antarctolima) ferraziana sp. nov.), Bryozoa, Polychaeta (serpulid tubes) and Echinodermata. Specimens occur in debris flows deposits of the Low Head Member, as part of a fan delta setting in a high energy, shallow marine environment. Liothyrella sp., Adamussium auristriatum sp. nov. and Limatula ferraziana sp. nov. are among the oldest records for these genera in King George Island. In spite of their restrict number and diversification, bivalves and brachiopods from this study display an overall dispersal pattern that roughly fits in the clockwise circulation of marine currents around Antarctica accomplished in two steps. The first followed the opening of the Tasmanian Gateway at the Eocene/Oligocene boundary, along the eastern margin of Antarctica, and the second took place in post-Palaeogene time, following the Drake Passage opening between Antarctic Peninsula and South America, along the western margin of Antarctica.

Shell beds from the Low Head Member (Polonez Cove Formation, early Oligocene) at King George Island, west Antarctica: new insights on facies analysis, taphonomy and environmental significance

Abstract Shell bed levels in the Low Head Member of the early Oligocene Polonez Cove Formation at King George Island, West Antarctica, are re-interpreted based on sedimentological and taphonomic data. The highly fossiliferous Polonez Cove Formation is characterized by basal coastal marine sandstones, overlain by conglomerates and breccias deposited in fan-delta systems. The shell beds are mainly composed of pectinid bivalve shells of Leoclunipecten gazdzickii and occur in the basal portion of the Low Head Member. Three main episodes of bioclastic deposition are recorded. Although these shell beds were previously interpreted as shelly tempestites, we present an alternative explanation: the low fragmentation rates and low size sorting of the bioclasts resulted from winnowing due to tidal currents (background or diurnal condition) in the original bivalve habitat. The final deposition (episodic condition) was associated with subaqueous gravity driven flows. This new interpretation fits with the scenario of a prograding fan-delta front, which transported shell accumulations for short distances near the depositional site, possibly between fair-weather and storm wave bases. This work raises the notion that not every shell bed with similar sedimentological and taphonomic features (such as geometry, basal contact, degree of packing and shell orientation in the matrix) is made in the same way.

A (tidal-marine) boulder pavement in the late Cenozoic of Seymour Island, West Antarctica: contribution to the palaeogeographical and palaeoclimatic evolution of West Antarctica

Abstract We report here the discovery of a boulder pavement cropping out at the base of the Hobbs Glacier Formation (Miocene), on the northern sector of Seymour Island (Isla Marambio), West Antarctica, along the contact with the underlying La Meseta Formation (Eocene). The feature described has many points in common with boulder pavements developed in tidal-marine environments. The clasts of the pavement are mostly boulders and bear up to three sets of glacial striae on their bevelled truncated surfaces, but are not elongate parallel to them or bullet shaped. No diamictite body was identified associated with the boulder pavement. These features differ from those of boulder pavements described from terrestrially glaciated Cenozoic deposits and may indicate a shallow glaciomarine environment for the late Cenozoic of Seymour Island.

Chapter 50 Moema laminites: a newly recognized Neoproterozoic (?) glaciogenic unit, São Francisco Basin, Brazil

Abstract A recently identified diamictite and silt-clay laminite, which discordantly overlie the Archaean basement and underlie the Neoproterozoic Bambui Group, have been informally named as the Moema Laminites. They are preserved at the southwestern margin of the São Francisco Basin in southeastern Brazil, and are widely distributed in the central-western Minas Gerais state. They crop out discontinuously over at least 140 km along a north–south direction. The nomenclature of and stratigraphic relationships between the Moema Laminites and other isolated Neoproterozic occurrences of similar rocks are in a state of flux. Two exceptionally good exposures of the Moema Laminites show good evidence for deposition under glacial conditions. At the Formiga locality, a single glacial advance is registered by a deformation tillite, while overlying laminite records deposition in a post-glacial, probably marine basin following deglaciation. At the SAFFRAN quarry, striations on a bedding plane may have been caused by floating sea-ice that just touched the bottom of the basin. Much additional work is needed to establish relationships between the Moema Laminites and other similar occurrences. If these and Moema Laminites are shown to be Cryogenian glacial deposits, the area covered by the Cryogenian glaciations in the São Francisco basin is much larger than formerly believed.