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Keywords:

  • Columbus Basin;
  • growth faults;
  • hummocky cross-stratification;
  • Mayaro Formation;
  • palaeo-Orinoco Delta;
  • shelf-edge deltas;
  • swaley cross-stratification

Abstract

Pliocene age deposits of the palaeo-Orinoco Delta are evaluated in the Mayaro Formation, which crops out along the western margin of the Columbus Basin in south-east Trinidad. This sandstone-dominated interval records the diachronous, basinwards migration of the shelf edge of the palaeo-Orinoco Delta, as it prograded eastwards during the Pliocene–Pleistocene (ca 3·5 Ma). The basin setting was characterized by exceptionally high rates of growth-fault controlled sediment supply and accommodation space creation resulting in a gross basin-fill of around 12 km, with some of the highest subsidence rates in the world (ca 5 to 10 m ka−1). This analysis demonstrates that the Mayaro Formation was deposited within large and mainly wave-influenced shelf-edge deltas. These are manifested as multiple stacks of coarsening upward parasequences at scales ranging from tens to hundreds of metres in thickness, which are dominated by storm-influenced and wave-influenced proximal delta-front sandstones with extensive, amalgamated swaley and hummocky cross-stratification. These proximal delta-front successions pass gradationally downwards into 10s to 100 m thick distal delta front to mud-dominated upper slope deposits characterized by a wide variety of sedimentary processes, including distal river flood and storm-related currents, slumps and other gravity flows. Isolated and subordinate sandstone bodies occur as gully fills, while extensive soft sediment deformation attests to the high sedimentation rates along a slope within a tectonically active basin. The vertical stratigraphic organization of the facies associations, together with the often cryptic nature of parasequence stacking patterns and sequence stratigraphic surfaces, are the combined product of the rapid rates of accommodation space creation, high rates of sediment supply and glacio-eustasy in the 40 to 100 Ka Milankovitch frequency range. The stratigraphic framework described herein contrasts strikingly with that described from passive continental margins, but compares favourably to other tectonically active, deltaic settings (for example, the Baram Delta Province of north-west Borneo).