Style and stages of valley fill aggradation-incision cycles in the Northern Hill Range, Kachchh, Western India

Abstract

Sedimentary sequences often provides ample of clues to reconstruct the past climatic as well as tectonic perturbations undergone by a region, however owing to similar residual products the differentiation is an intricate issue. We report the aggradation and incision phases along the Northern Hill Range (NHR), Kachchh; one of the most seismically active, but poorly explored, intraplate environments on earth. The northerly draining fluvial sequences in the NHR, exhibit a variety of valley fill, channel fill and strath terrace sequences over the hanging wall block of one the most seismically active reverse faults in the world ∼ Kachchh Mainland Fault (KMF). The study of these sequences owing to its position, enables us to reconstruct the uplift history of the KMF as well as scrutinize the signatures of climatic and tectonic events in sedimentation since the last glacial period. Based on detailed stratigraphy and OSL chronology, we identified five cyclic phases of aggradation and incision during the last 24 ka period. The NHR experienced two aggradation phases during 24–10 ka and 8–4 ka period in response to climate change. Simultaneously, the KMF experienced at least two phases of uplift 1) > 24 ka and <4 ka, which led to landscape modification at regional scale inform of terrace formation. Global events such as Early Holocene Optima and 4.2 ka arid phase are also archived in form of strong erosional phase in the fluvial sequences of NHR. Although due to paucity of archives and discreet nature of sedimentary succession in dryland environments, the actual causal mechanism for the incision at 4 ka period remains ambiguous. The longterm minimum average uplift rate of the eastern KMF varies from 0.1 to 1.0 mm/a, which is directly linked to the longterm deformation rate, often experienced in such intraplate setting. The study exemplifies the fluvial response, of transport deficit dryland environments, to external perturbations and their application in reconstructing palaeo extreme events.