Initial cyclostratigraphy of the middle Nama Group (Schwarzrand Subgroup) in southern Namibia

Abstract

The Ediacaran Period includes critical evolutionary trends of early complex life as well as climatic variations associated with ocean oxygenation, glaciations, and carbon cycling, which are still poorly understood in terms of astronomical climate forcing. The middle Nama Group in southern Namibia was deposited during the late Ediacaran in mainly shallow marine environments within a foreland basin and consists of hierarchically arranged depositional sequences. Here, we test a possible astronomical origin of these sedimentary variations by developing an initial cyclostratigraphic framework based on satellite images integrated with recently published high-precision U-Pb zircon ages. Regular sedimentary alternations occur dominantly on scales of several tens of meters, accompanied by smaller- and larger-scale variations, and are correlatable over distances of ∼50 km. Throughout the studied succession, 35 to 39 alternations are recognized on this dominant scale, which have an average duration of ∼120–180 k.y. This duration corresponds well with the period of short eccentricity (∼100 k.y.), given the likely presence of hiatuses, or alternatively, the period of obliquity amplitude modulation (∼173 k.y.), which would imply no time is missing on this scale. The dominant alternations are consistent with previously identified medium-scale sequences in this succession, which have been interpreted to record fluctuations in relative or eustatic sea level. We hypothesize that astronomically-forced fluctuations in eustatic sea level modulated deposition of the middle Nama Group. Geochemical studies suggested a relation between fossil distribution, redox variability and sea level, implying that astronomical forcing may have played a role in the distribution of early complex life.