Cretaceous coastal lagoon facies: Geochemical insights into multi-stage diagenesis and palaeoclimatic signals

Abstract

Shallow-water Cretaceous, carbonate platform archives have only rarely been explored through their elemental signature. Yet, facies-sensitive geochemical records can shed light onto the specific dynamics of shallow/proximal settings, where changes in depositional conditions are pronounced. Changes stemming from early and later diagenetic processes must be considered, since they may alter the original geochemical signals, but still contribute to a better understanding of the evolution of ancient shallow carbonates.Here, the elemental record of a variety of carbonate materials from two middle Cenomanian coastal lagoon settings is examined. The two selected sections (Lousa and Lampas, W Portugal) provide a varied record of lithofacies, including matrix micrite, corresponding to diverse palaeoenvironments (marine, marine lagoon, restricted lagoon and subaerial exposed intervals), along with dolomitized beds and atypical darkened grey carbonate horizons. Sparry carbonate veinlets that intersect these materials are included for comparison. Based on the characteristic elemental composition of each carbonate material, and by means of a stepwise statistical approach, dolomitization was determined to be a rather early feature occurring exclusively along shallow to frequently exposed intervals. In contrast, carbonate veinlets were identified as the latest diagenetic event. Matrix micrite and darkened beds present the lowest degree of geochemical differentiation, though certain key elemental contrasts allow for their comparison. Accordingly, the atypical case of darkened beds was attributed to an early blackening process in which low contents of organic matter and non-carbonate mineral phases played a decisive role. Matrix micrite samples—presenting a rather narrow range of elemental fluctuations—were linked to previously identified palaeoenvironmental settings by means of stratigraphically constrained cluster analysis. Specific combinations of elemental trends identified for each established cluster reflect variations in palaeoclimatic regimes throughout the studied interval. Furthermore, the degree of connectivity between open waters and coastal lagoons emerged as predictive of the distribution of major and trace elements in ancient shallow carbonates.This study explores the depositional and diagenetic controls forcing elemental distribution across dynamic shallow carbonate settings, as is the case of coastal lagoons. Complex and atypical elemental fingerprints, established by means of statistical analysis, provide arguments to reconstruct the evolution of the examined carbonate materials. Our findings are highly pertinent, as they: (i) provide a rich dataset not only regarding Lusitanian Basin deposits, but also for coeval settings where basin evolution and diagenesis are pivotal; (ii) underline the insight to be gained from time series analyses of raw elemental data collected from carbonate facies; and (iii) place a suite of numerical analyses within a logical hierarchy for stratigraphic facies analysis, fully supported by independent data.