Disentangling orbital, sub-orbital, and tectonic signatures from lacustrine sediments developed upon a half-graben (Lake Ifrah Basin, Northwest Africa): Insights into lowest-rank T-R sequences in low accommodation basins

Abstract

Lacustrine settings constitute a unique environment that preserves detailed expressions of allocyclic signals such as those of climate and tectonics. Possible decryption of these signatures stems from careful scrutiny of the sedimentation dynamics (temporary base-level variations), lake-level fluctuations (accommodation), and resulting strata bounding surfaces that are used to build a conventional sequence stratigraphic framework. However, due to discrepancies between marine, to which this approach has been initially dedicated, and lacustrine settings (especially regarding the physical scale), deciphering climate from tectonic forcing becomes unwieldy in such interior basins. The present work deals with this challenge and provides insights from a case study where lacustrine sedimentation occurs on a tectonically active half-graben within a key climate region (Lake Ifrah, Northwest Africa). We conducted conventional sedimentological and high-resolution sequence-stratigraphic analyses, integrated with palaeolimnological proxies (geochemical elements and ostracod species). Up to five facies models (accounting for lithological domination, wind-driven energy, and lake-level state) and three lowest rank T-R sequences, deposited since the Marine Isotope Stage-3 (MIS-3), have been identified. Periods with sustained high lake levels appear to be mainly precession-paced (as during MIS-3 and the Early Holocene), although the role of obliquity is shown to influence the hydrological budget as well. Furthermore, sedimentation dynamics are shown to respond to millennial timescale climate variability associated with North Atlantic cooling events (Dansgaard-Oeschger stadials, Heinrich Events) and, interestingly, to enhanced Saharan winds during the deglacial period. On the other hand, tectonism had a rather instantaneous effect on lake level and sedimentation. Two tectonic pulses marking instantaneous differential hanging-wall subsidence have triggered a sharp drop in relative lake level, hence conditioning a forced regression.We highlight the importance of the conventional high-resolution sequence stratigraphy in shaping our understanding of the cyclic interplay between orbital/sub-orbital and tectonic forcings, so as the resulting sedimentation dynamics and lake-level cycles in lacustrine settings. We stress the role of the forced regression concept and associated systems tract and bounding surfaces, as well as the importance of using ostracods and geochemical proxies to trace transgressions and cryptic surfaces with sequence-stratigraphic significance, such as the maximum flooding surface, in lacustrine settings.