Pablo J. Pazos

The Late Carboniferous Glacial to Postglacial Transition: Facies and Sequence Stratigraphy, Western Paganzo Basin, Argentina

Abstract Namurian-early Westphalian glacial-postglacial deposits have been documented in the Paganzo Basin, Argentina, but integrated sequence stratigraphic and sedimentological analyses of the glacial-postglacial transition are rare. In the Cuesta de Huaco section the glacial-postglacial transition encompasses the lower sequence of the existing schemes. Glacioeustatic changes in sea-level controlled the architecture and vertical arrangement of facies. The basal sequence boundary is coeval with the onset of glaciation when sea level fell, contemporaneous with glacial erosion and bypass of sediment. The upper sequence boundary is a high-frequency sequence boundary. The basal deposits are interpreted as an early transgressive systems tract (ETST) initiated during glacial retreat. The interval fines upward, contains current reworked diamictites, conglomerates and sandstones deposited in a subglacial to proglacial environment (Facies I-II) and ice-rafted glaciomarine rhythmites (Facies III). Postglacial brackish-marine black shales (Facies IV) represent the late transgressive systems tract (LTST). Micritic-limestone beds were deposited during a period of sediment starvation coeval with the maximum marine inundation in the basin (MFZ). It marks a turnaround in the stacking pattern that became coarsening and thickening upward during the overlying highstand systems tract (HST). It consists of fine-grained turbidites (Facies V) and classical turbidites with wave reworked beds at the top (Facies VI) deposited in a shallow deltaic systems. Intervening postglacial brackish deposits contain an ichnofauna dominated by trackways of arthropods and minor grazing traces, preserved in bedding planes, that resemble nonmarine ichnocoenoses and differ from the archetypal brackish ichnofaunas, even when palynological data support the saline-depleted environment.

The Record of the Varanger Glaciation at the Río De La Plata Craton, Vendian-Cambrian of Uruguay

A sedimentary succession included in the lower section of the Playa Hermosa Formation from the Playa Verde Basin, Uruguay, is described. Two facies associations, one mainly coarse- to medium-grained and other one fine-grained, have been defined (FA I-II). In the first one, breccias, conglomerates, sandstones and minor mudstones were deposited in a subaqueous depositional setting (proximal) suggesting slope instability and resedimentation processes. The second one contains diamictites, rhythmites, sandstones and mudstones and presents abundant evidence of soft-deformation, also interpreted to be deposited in a subaqueous environment (distal). Dropstones, clast layers, diamictites, rhythmites and varve-like deposits are interpreted as ice rafting processes generated during a glacial episode. This glacial-related succession constitutes the first record from the Varanger glaciation at the Rio de la Plata Craton of the late Neoproterozoic age and also represents one of the oldest sedimentary records after the collision of the Rio de la Plata and Kalahari Cratons. A combined interaction of extensional faulting and glaciation in a tectonically active basin with locally high subsidence rates, resulted in high rates of sedimentation and resedimentation processes. As a whole, the sedimentary succession sets a relevant datum to be used in future paleogeographic reconstructions of the Vendian glacial record in southern South America.

Protovirgularia dichotoma—Protovirgularia rugosa: An Example of a Compound Trace Fossil from the Lower Cretaceous (Agrio Formation) of the Neuquén Basin, Argentina

This study focuses on the description of a compound trace fossil from the Cretaceous of the Neuquén Basin, Argentina. This structure comprises two discrete ichnotaxa: Protovirgularia cf. dichotoma M’Coy, 1850, and Protovirgularia cf. rugosa (Miller and Dyer, 1878). Open nomenclature is chosen due to some differences from the ichnospecies diagnoses. The compound trace as a whole is the product of locomotion and resting behaviors (repichnia and cubichnia); this is not questioned but discussed. The inferred producer is a deposit-feeding palaeotaxodont bivalve, which belongs to a taxonomic group almost unknown and not yet reported in the Neuquén Basin. The sample was taken from rocks interpreted as being deposited in a tide-influenced, marginal-marine setting, corresponding to the uppermost section of the Agrio Formation. This also represents one of the first cases of a compound trace fossil described for the Neuquén Basin.

Chapter 52 The Playa Hermosa Formation, Playa Verde Basin, Uruguay

Abstract The Playa Hermosa Formation (Fm.) (Playa Verde Basin) is a volcano-sedimentary unit that crops out in the extreme south of the Dom Feliciano Belt in Uruguay. This formation has traditionally been interpreted as non-glacial in origin, but recently it has been suggested that the lower part at least may be glacially influenced. The stratigraphic position of the Playa Hermosa Fm. and its correlation with other Neoproterozoic units of Uruguay, Argentina and southern Brazil remains in dispute. An age of c. 580 Ma is indicated by a hornblende 39Ar/40Ar age on magmatism regarded as coeval with syn-sedimentary volcanism. Preliminary palaeomagnetic data suggest a primary remanence and a mean geomagnetic pole consistent with the proposed apparent polar wander path for the Río de La Plata craton. In combination, these data suggest mid-Ediacaran glacial sedimentation in low to intermediate latitudes on the Río de la Plata craton. However, this conclusion needs to be reinforced by more thorough studies of the age and origin of the Playa Hermosa Fm.