Event-based deposition in the ever-emptying Yatir Reservoir, Israel

This study involves identification, correlation and characterization of sedimentary event couplets deposited by runoff events entering a reservoir draining Nahal Yatir, a small drainage basin in the semiarid southern Hebron Mountains, Israel. Textural analyses of samples of an individual event layer show the spatial and the relative temporal intra-event variations that occur in the texture of sediment deposited in an ever-emptying setting. An event layer is comprised of a generally ungraded sub-parallel or cross laminated basal singlet. The upper singlet is a parallel laminated medium silt grading to clay. Event couplet boundaries are distinct and allow correlation almost all over the reservoir. The spatial homogeneity of the basal unit implies that it was formed by cessation of traction load as the reservoir filled. The congruence of the upper singlet is evidence of settling from suspension. Hence, two distinct depositional processes operate in an ever-emptying reservoir: cessation of traction load occurs distally and progressively at more proximal locations, whereas settling from suspension occurs everywhere almost contemporaneously. The distinction between ever-emptying and ever-full reservoirs and lakes may be used to postdict palaeohydrological regimes of individual sediment transporting events.