High-frequency stacking pattern and stages of canyon/gully evolution across a forced regressive shelf-edge delta-front

Abstract

The succession of varying facies tracts filling within a canyon, or a deep incised gully, cutting across the delta-front lobes at the shelf-edge requires to be explained by high-frequency allocyclic and/or autocyclic events. A ‘fifth- and sixth-order’ sequence-stratigraphic model for the shelf-edge canyon-fill or gully-fill is here proposed for the first time. The first Gelasian cold phase and the renewal of transpressional uplift of basin-margin occurring at the same time enforced regressive transit of the inner-shelf paleo-Orinoco delta onto the paleo-shelf-edge. Its delta-front developed along the shoulders of the major successive basin-bounding growth faults. The delivery of clastic sediments into the deeper slope and beyond in the Columbus Basin was through the gullies and canyons that incised and cannibalized across the shelf-edge delta. The Mayaro Formation of southeast Trinidad furnishes the role of growth faulting during the first phase of delta-front development at the paleo-shelf-edge. Thereby, it serves as an example of several ‘fifth-order sequences’, within a ‘fourth-order’ equivalent episode, which is contemporaneous to both the glacio-eustatic episode, the Pretiglian, and the renewal of transpressional uplift after a pause during the Pliocene. Thus, large incision and subsequent filling within the Mayaro Formation cover a passage of time (‘fifth-order’ equivalent) that is unequivocally shorter than the span of the falling Pretiglian eustatic sea-level and second renewed phase of transpression tectonism. The categorization of sedimentary facies of the canyon-fill or gully-fill and the encasing delta-front outside the canyon/gully cut into a series of ‘sixth-order’ genetic facies tracts shows variations of geomorphic stages of the canyon/gully evolution – from accommodation creation to the stages of filling. These stages, therefore, should be attributed to the ‘sixth-order’ equivalent autogenic changes influenced by high-frequency growth-tectonics, instead of assuming the stages to be related to the relatively lower-frequency glacio-eustatic allocyclicity of the earliest Gelasian.