Controls on lacustrine carbonate mineralogy and cyclicity

: Insights from hydrogeochemical modelling and application to the BVF pre-salt

Abstract

Lacustrine carbonate deposition is sensitive to changes in climate and tectonics that influence hydrology and hydrochemistry, and lake and catchment morphology. Despite a decade of extensive research, debate of all these factors continues regarding the conditions under which the Barra Velha Formation (BVF) carbonates and Mg-clays were deposited during early Cretaceous rifting that formed the South Atlantic (123-115 Ma). Understanding of BVF mineralogy (carbonates, Mg-clays, and silica) and cyclicity can be improved through geochemical modelling of the soda-type (Na-CO3) inflow and lake waters from which the BVF most likely precipitated. Models of progressive evaporation confirm that waters of varied chemical composition can sustain the precipitation of a BVF-like assemblage at lake water salinities sometimes >400 g/L TDS. No individual climatic, geologic, or morphologic parameter beside chemistry can accurately predict the mineral assemblage that will precipitate from waters assessed across a range of saline lake systems. Sediment cyclicity can be generated in models of kinetically controlled mineral precipitation without altering climatic conditions or water chemistry, but all models of a given chemistry will converge on the same mineral assemblage unless the climate and or chemistry is changed. Process-based algorithms can be used to drive cyclic changes in kinetically controlled mineral precipitation in a balanced hydrogeochemical model that only defines explicitly the rates of evaporation and rainfall over the lake. The total volume of mineral precipitation at each model step shows the expected increase with falling lake depth, associated with increasing lake water TDS. The mineralogy precipitated is determined by the rate of change in lake depth. This is in direct contrast to the conceptual models that attribute BVF facies cyclicity to changes in absolute lake depth. Further investigation with this process-based hydrogeochemical modelling approach will likely yield more results that question our current understanding of ancient and modern lacustrine carbonate deposition.

Date of Award	18 Mar 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Fiona F Whitaker (Supervisor)