Abstract

Marine deposits that crop out in the Guadix Basin (Spain) comprise three upper Tortonian stratigraphic units that are bounded by unconformities representing tectonic and/or eustatic events. The two upper units (II and III) have been interpreted as highstand and lowstand systems tracts, respectively. This work focuses on Unit III, which consists of two shallow-marine depositional systems: one a Gilbert-type delta on the basin margin and the other a shelf system with local reefal patches in its center. Both of these systems lie over slope and deep pelagic basinal depositional systems of Unit II. The bathymetric difference between Units II and III indicates a relative sea-level fall related to a rapid stage of tectonic uplift of the basin. This sea-level fall produced a displacement of the coastline toward the basin interior, resulting in a tectonically induced forced regression, and provided the context for deposition of lowstand systems tract of Unit III.

A detailed analysis of the deltaic depositional system of Unit III reveals at least five progradational phases represented by sigmoidal conglomerate lithosomes bounded by erosional surfaces. Overall, these lithosomes are characterized by a reduction in thickness of foresets and by upward-decreasing distances between successive topsets and bottomsets. The five phases of deltaic progradation can be correlated with five phases of aggradation in the time-equivalent shelf depositional system, each of which is represented by thickening- and coarsening-upward successions (TCU); these successions comprise a stacking of marl-sand cycles. The thickness of the TCU successions decreases upward, coinciding with the upward decrease in thickness in the deltaic system. Taking into account the dominant basinward displacement of the brinkpoints of the deltaic clinoforms, with a slight upward component, we have deduced that the progradation of Unit III took place in a context of slightly rising relative sea level and represents a normal regression.

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