Recognizing seasonal fluvial influence in ancient tidal deposits

Abstract

AbstractSeasonal changes in rainfall affect water and sediment discharges in fluvial systems. This is reflected within the rock record by changes in (1) the flow processes and structures created and (2) the composition and volume of sediment. This study demonstrates that high-resolution sedimentological studies of rhythmites in ancient tidal bar deposits reveal a seasonal fluvial influence. The rocks studied belong to the Plio-Pleistocene Erin Formation on Trinidad, deposited under tropical seasonal conditions in the fluvial–tidal transition zone and tidally dominated palaeoenvironments. Time-series and stratigraphic analyses were undertaken of two rock subsections separated by an erosional surface. Spring–neap, monthly and semi-annual cycles were identified within the deposits. Sand-rich and mud-rich couplets showed neap–spring cyclicity, with gradual transitions between flaser, wavy and lenticular bedding. Seasonal changes in fluvial discharge were recognized by minor scours that marked periods of relatively higher discharge. The content and distribution of organic material varied through the seasonally influenced cycle. Rare and dwarfed trace fossils and bioturbated beds also showed a periodic pattern, interpreted as representing changes in palaeoenvironmental conditions due to seasonal freshwater discharge. As a result, reservoirs containing similar deposits may have porosity and permeability variations with seasonal depositional periodicity.Supplementary material: The full set of measured time-series data is available at https://doi.org/10.6084/m9.figshare.c.3283376