LPIA palaeo-fjords: new insights from northern Namibia

A detailed understanding of the features produced beneath ice sheets, in terms of both their spatial distribution and their genesis, is crucial for a correct reading of the glacial record. An assemblage of features cut into crystalline bedrock in northern Namibia were produced approximately 300 myr ago during the Late Paleozoic Ice Age and exhibit an outstanding array of structures contained within deep palaeo-fjords, capped by glaciogenic rocks of the basal Dwyka Group. With the exception of a handful of recent studies in the last 3 years, this area has been largely ignored since the mid-twentieth century. By utilizing large-scale aerial and close-range photogrammetry, we map the subglacial substrate in the Eastern Hoarusib palaeovalley at an unprecedented resolution, marking the first time that large swarms of striae and other subglacial bedforms, such as crescentic markings, have been mapped. Quantitative analyses of macroscale ice flow features include around 17 000 measurements, reducing uncertainty regarding ice flow direction. In the Gomatum palaeovalley, c. 100 km to the SW, we utilize detailed micromorphology and sedimentological analyses to examine basal Dwyka Group strata, recognizing delicate dropstone-bearing laminae and micro-imbrication in heterolithic deposits (fining-upward sandstones and siltstones, rhythmic couplets) lying approximately 50–60 m above the present valley floor. These deposits are interpreted as ice-rafted debris and unidirectional current deposits, which testify to the presence of lingering ice during the transgression of the palaeovalleys. More widely, we argue that the quantitative photogrammetric approaches showcased herein could be more widely adopted in studies of the deep time glacial record and possibly benefit the research of Quaternary fjord systems.