Australia: The Land Where Time Began

A biography of the Australian continent 

Western Tethys – Glacial Dropstones during the Late Aptian-Early Albian Cold Snap – Palaeoclimate and Palaeogeographic Implications for the Mid-Cretaceous

According to Rodríguez-López et al. it is commonly thought that the only interval of the history of the Earth when there was a cold climate but from which no direct evidence of polar ice has been recovered was the Late Jurassic-Early Cretaceous. In this paper a recently described interval from the subtropical Iberian Basin (western Tethys) that contained a dropstone is described which provides evidence of contemporaneous polar glaciation. Laterally, the interval is correlated for 4.8 km, and contains a boulder and 2 quartzite dropstones of cobble size, that are encased in fissile black shales and fin-grained sandstones dating to the mid-Cretaceous. The dimensions of similar large clasts that have been published previously it is suggested that the only glacial dropstones and impact ejecta blocks reach the dimensions of the boulder-sized dropstone that has been reported from Iberia. There are morphological features on the dropstones that are compatible with transport in a glacial setting and abrasion in a subglacial setting resembling closely the features that are observed on boulders that have been exposed near the snouts of Iceland glaciers. The dropstones recovered from Spain dating to the Late Aptian correlate with many other similar erratics recovered from the Northern Palaeohemisphere and the Southern Palaeohemisphere, which suggests that during certain periods of the Early Cretaceous ice sheets formed around the Palaeonorth Pole. It is suggested by the results of this study, together with evidence from associated evidence, such as the occurrence of glendonites, tillites, moderate- to high-amplitude sea level oscillations from around the world, minimum partial pressures of atmospheric CO₂, variation in assemblages of nannofossils from low and high latitudes and isotopic excursions, that there were periods of ice growth and decay during the mid-Cretaceous that had an influence on the palaeotemperature, palaeoecology and sedimentology of the marine realm. Recent results from Arctic Canada that indicate cool shelves, and a cold snap during the mid-Cretaceous, that developed for about 6 Myr between 118 Ma and 112 Ma, support the new data from Iberia. The Late Aptian dropstones reported from eastern Iberia are suggested by Rodríguez-López et al. to have likely been transported from high northern latitudes towards subtropical latitudes in the western Tethys by an extreme iceberg drift similar to those that occur in the modern Atlantic Ocean. Rodríguez-López et al. suggest icebergs that had been released from a fringing ice sheet in the north may have travelled south through the Greenland-Norwegian Seaway.

Conclusions

The majority of dropstones dating to the Aptian-Albanian originating in the palaeonorthern Palaeohemisphere accumulated between 65^oN and 25^oN, along the Greenland-Norwegian Seaway. In this paper Rodríguez-López et al. report the southernmost examples from the Northern Hemisphere dating to the late Aptian-early Albian. A dropstone of boulder size displays clear glacial features, and the encasing material is fine-grained laminated coastal deposits of the Iberian Basin. This boulder, as well as 2 other glacial dropstones of cobble size, all correlated with boulder sized dropstones dating to the late Aptian-Early Albian from Svalbard, Arctic Canada and Europe in the Northern Hemisphere, as well as those from Australia. According to the interpretation of these dropstones they may have been deposited from icebergs which had overturned that had been released from the Arctic ice sheet.  Rodríguez-López et al. suggest that as such an ice sheet occurred at the same time and location as the High Arctic Large Igneous Province (HALIP), it is possible that interaction between ice and magma may have triggered more extensive and larger iceberg generation than at any other times.

Sources & Further reading

Rodríguez-López, J. P., C. L. Liesa, G. Pardo, N. Meléndez, A. R. Soria and I. Skilling (2016). "Glacial dropstones in the western Tethys during the late Aptian–early Albian cold snap: Palaeoclimate and palaeogeographic implications for the mid-Cretaceous." Palaeogeography, Palaeoclimatology, Palaeoecology 452: 11-27.