Gondwanan Ice Sheets Research Articles

Basin fill evolution and paleotectonic patterns along the Samfrau geosyncline: the Sauce Grande basin–Ventana foldbelt (Argentina) and Karoo basin–Cape foldbelt (South Africa) revisited

As integral parts of du Toit’s (1927) “Samfrau Geosyncline”, the Sauce Grande basin–Ventana foldbelt (Argentina) and Karoo basin–Cape foldbelt (South Africa) share similar paleoclimatic, paleogeographic, and paleotectonic aspects related to the Late Paleozoic tectono-magmatic activity along the Panthalassan continental margin of Gondwanaland. Late Carboniferou-earliest Permian glacial deposits were deposited in the Sauce Grande (Sauce Grande Formation) and Karoo (Dwyka Formation) basins and Falkland–Malvinas Islands (Lafonia Formation) during an initial (sag) phase of extension. The pre-breakup position of the Falkland (Malvinas) Islands on the easternmost part of the Karoo basin (immediately east of the coast of South Africa) is supported by recent paleomagnetic data, lithofacies associations, paleoice flow directions and age similarities between the Dwyka and the Lafonia glacial sequences. The desintegration of the Gondwanan Ice Sheet (GIS) triggered widespread transgressions, reflected in the stratigraphic record by the presence of inter-basinally correlatable, open marine, fine-grained deposits (Piedra Azul Formation in the Sauce Grande basin, Prince Albert Formation in the Karoo basin and Port Sussex Formation in the Falkland Islands) capping glacial marine sediments. These early postglacial transgressive deposits, characterised by fossils of the Eurydesma fauna and Glossopteris flora, represent the maximum flooding of the basins. Cratonward foreland subsidence was triggered by the San Rafael orogeny (ca. 270 Ma) in Argentina and propogated along the Gondwanan margin. This subsidence phase generated sufficient space to accommodate thick synorogenic sequences derived from the orogenic flanks of the Sauce Grande and Karoo basins. Compositionally, the initial extensional phase of these basins was characterized by quartz-rich, craton-derived detritus and was followed by a compressional (foreland) phase characterized by a paleocurrent reversal and dominance of arc/foldbelt-derived material. In the Sauce Grande basin, tuffs are interbedded in the upper half of the synorogenic, foldbelt-derived Tunas Formation (Early–early Late? Permian). Likewise, the first widespread appearance of tuffs in the Karoo basin is in the Whitehill Formation, of late Early Permian (260 Ma) age. Silicic volcanism along the Andes and Patagonia (Choiyoi magmatic province) peaked between the late Early Permian and Late Permian. A link between these volcanics and the consanguineous airborne tuffs present in the Sauce Grande and Karoo basins is suggested on the basis of their similar compositions and ages.

Speculations on the controls on cyclic peritidal carbonates: ice-house versus greenhouse eustatic controls

Metre-scale peritidal cyclothems, which are distinctive components of Lower Palaeozoic and many Mesozoic platform carbonates, are not common in Carboniferous sequences. In their place the platform sequences are characterized by 5–10 m thick, largely subtidal limestones, with prominent exposure surfaces. The former type of platform cyclothems developed during ‘greenhouse’ phases when, in the absence of major continental ice sheets, sea-level oscillations were caused by small-scale, obliquity and precession Milankovitch rhythms. Under such conditions platforms rapidly aggraded with the deposition of peritidal cyclothems. The Carboniferous was an ‘ice-house’ phase when the waxing and waning of Gondwanan ice sheets, caused by eccentricity rhythm-generated climatic changes, created high-amplitude, rapid falls and rises in sea level. The resulting carbonates reflect such changes with ‘catch-up’, subtidal-dominated sequences and pronounced subaerial exposure surfaces capping the cyclothems.

Carboniferous

Abstract Carboniferous geology has developed very rapidly over the past two decades since ideas on sedimentology and new radiometric scales, together with new biostratigraphical schemes based on miospores, conodonts and foraminifera have been synthesized with ideas on plate tectonic processes, climatic changes and oscillations of sea level. The application of the McKenzie model of basin development to the Carboniferous, and the relating of eustatic sea-level changes, which had a profound effect on Carboniferous palaeo-geography, to fluctuations in the size of the Gondwanan ice sheets, have been major factors in the understanding of Carboniferous processes. The Carboniferous evolution of Britain took place to the (present) north of a plate suturing occurring along a line extending from Galicia, through Armorica and the Massif Central to the Vosges, during mid Devonian to early Carboniferous times. In front of this collision zone was the Rheno-Hercynian zone which opened probably during mid Devonian times and which was partly floored by oceanic crust; its closure can be traced through to late Carboniferous times as a series of northwardly directed thrust complexes. To the (present) north of this zone was a foreland on which basins developed on either side of a persistent Wales-London-Brabant High. Studies by Dewey (1982) and Leeder (1982, 1987, 1988) have shown that late Devonian to early Carboniferous extension, involving β factors of up to 2, produced a series of grabens and half-grabens in the Caledonian basement. These developing structures were the dominant control on early Carboniferous sedimentation and produced a sea-bed topography which was

Climate model comparison of Gondwanan and Laurentide glaciations

Geologic studies indicate that the Carboniferous glaciation on Gondwanaland was approximately as extensive as the ice sheets during the Pleistocene. However, there is one major difference between the climate boundary conditions for the two ice sheets: the Gondwanan ice sheet was located on a supercontinent. Three different levels of sensitivity experiments were conducted to examine the effect of the large landmass on the magnitude of summer warming over the ice sheets: (1) simulations with present solar luminosity and present orbital forcing resulted in summer temperatures over the Gondwanan Ice Sheet 12°–17°C greater than over the Pleistocene Laurentide Ice Sheet; (2) lowering the solar constant 3% or modifying the seasonal insolation cycle to a “cold summer orbit” reduced the warming but still led to significant differences between Gondwanan and Pleistocene simulations; and (3) the combined effect of lowering the solar constant and modifying the seasonal insolation cycle to a cold summer orbit resulted in temperature patterns over the Gondwanan Ice Sheet similar to the Pleistocene. Model simulations also predict tropical sea surface temperatures about 4°C less than at present as a result of reduced solar luminosity. These results suggest that conditions necessary to explain Gondwanan ice sheet stability may be known, but required boundary conditions would be more extreme than in the Pleistocene. Although a number of uncertainties remain in these calculations, they help to better define critical conditions for glaciation for one of the most prolonged periods of continuous glaciation in Earth history.