Imprints of climate and neotectonic activity in lake sediments developed upon a reverse drag (Lake Ifrah, NW Africa): Integration of detailed sedimentological and sequence stratigraphic approaches.

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 may stem 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. The present work deals with this challenge and provides insights from a case study where lacustrine sedimentation occurs onto a tectonically active reverse drag associated with normal faulting within a key climate region (Lake Ifrah, Northwest Africa). Moreover, we investigated soft-sediment deformation structures (SSDSs) within Lake Ifrah sediments, which can be good proxies for neotectonic and paleoseismic activities. We analyzed both brittle and liquidization-induced SSDSs, including load structures, water-escape structures, shrinkage cracks, subsidiary grabens, and bed-parallel slips. 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). Furthermore, sedimentation dynamics are shown to probably 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. Facies analysis and deformation-trigger assessment helped to discern endogenic from exogenic triggers, with seismic activity inferred as a primary cause for liquidization-induced deformations.