Ice-rafted dropstones in “postglacial” Cryogenian cap carbonates

Dropstones of ice-rafted origin are typically cited as key cold-climate evidence in Cryogenian strata and, according to conventional wisdom, should not occur in postglacial, warm-water carbonates. In Namibia, the Chuos Formation (early Cryogenian) contains abundant dropstone-bearing intervals and striated clasts. It is capped by the Rasthof Formation, composed of laminites in its lower portion and microbial carbonates above. These laminites are locally found to contain pebble- and granule-sized lonestones in abundance. At the Omutirapo outcrop, meter-thick floatstone beds occur at the flanks of a Chuos paleovalley and are readily interpreted as mass-flow deposits. At Rasthof Farm, however, the clasts warp, deflect, and penetrate hundreds of carbonate laminations at both the outcrop and thin-section scale. We propose that these are dropstones, and we infer an ice-rafting mechanism. Evidence for vestigial glaciation concomitant with cap carbonate deposition thus merits a reappraisal of the depositional conditions of cap carbonates and their paleoclimatic significance.

A test of the efficacy of sand saltation for silt production: Implications for the interpretation of loess

Production of the silt that forms loess is attributed to processes operating in both glacial systems (glacial grinding) and sandy deserts (saltation-induced fracturing). However, the efficacy of saltation for significant silt production is controversial. Understanding the potential for silt production in deserts is essential for determining the paleoclimatic significance of loess. To better assess the significance of eolian abrasion for silt production, experimental abrasion was conducted in a device designed to simulate sand saltation at sustained storm-wind velocities (∼25 m/s). The design differs from previous work in (1) maintaining strong measured velocities for long duration, (2) removing preexisting silt and utilizing control samples, (3) and scaling results to estimate potential for loess accumulation. Scaling experimental rates of production to geologic proportions indicates that eolian abrasion of sand produces insufficient silt to create geologically significant loess deposits.

Paleo-Environmental Conditions, Paleoclimatic Significance and Effects of Weathering on Clay Deposits in the Lower Benue Trough, Nigeria. Mineralogical Approach

Combined methods for mineralogical identifications were used to characterise the clay deposits within the Lower Benue Trough of Nigeria to interpret paleo-environmental conditions, the paleoclimatic significance of the trough, and effects of weathering on the minerals as factors that favour its deposition/accumulation within the trough which host other important industrial minerals like coal, barite, limestone etc. Bulk-sample random-powder XRD data and data for clay fractions deposited onto zero-background quartz plates were measured. The samples contained kaolinite, vermiculite, and traces of smectite, and the non-clay phases included quartz, microcline, and muscovite. All samples were unaffected after glycolation, confirming the absence of significant smectite. Muscovite was characterized by the nature of its 10 Å basal peak with a width of <0.10° 2θ, which was very sharp. DTA/TGA results support the presence of kaolinite, and the characteristic kaolinite O-H, Al-OH, Si-OH and Si-O-Al FTIR bands also confirmed its presence. Vermicular and book-like morphologies were observed under the SEM, typical of kaolinitic clay from in situ alteration. High kaolinite abundance in these sediments is consistent with intense weathering of parent rocks rich in Al under wet/tropical paleo-climatic conditions with fresh and/or brackish water conditions in a continental setting. The variety of observed morphologies suggests that the deposits suffered more of chemical weathering. The clay deposits in Lower Benue Trough are quartz-rich, kaolinitic and derived from the chemical weathering of Al-rich source rocks.