Biosedimentary disturbances in shallow-water carbonate environments: An example from the Upper Hauterivian in the Lusitanian Basin

Abstract

An integrated analysis of the Hauterivian shallow-marine carbonates from the western margin of the Lusitanian Basin (Portugal) allows us to assess the paleoenvironmental significance of variations of biological, mineralogical, chemical and magnetic markers in a lagoonal environment. The Guincho section is characterized by nine facies included in different environments ranging from terrestrial and restricted lagoon to high-energy open platform. The variations of the different markers show a clear cyclic pattern. Certain curves are relatively well correlated, thus exhibiting a parallel trend, while others are anti-correlated. In this shallow water context, there is relation between the habitat [biocoenosis] and concentration of benthic foraminifera, the presence and activity of boring and micritizing organisms, the detrital input expressed by magnetic susceptibility (MS) as well as the chemical and isotopic composition of the carbonate fraction (especially K content and δ 13C). In this lagoonal environment, the increasing detrital input appears to reduce the relative density of infaunal benthic foraminifera in favor of epifaunal benthic foraminifera. These conditions lead to the development and prevalence of large benthic foraminifera with agglutinated tests, more particularly Choffatella decipiens. At the same time, there is a reduced activity of organisms, such as endolithic algae and cyanobacteria, which cause micritization of the grains; as a result, the δ 13C decreases while the magnetic susceptibility (MS) and K content increase. This detrital input increases the water turbidity, which modifies the faunal composition, which in turn has an effect on photosynthesis activity. In the studied area, the positive δ 13C excursion is associated with enhanced carbonate accumulation and production, while a negative shift in δ 13C is recorded during episodes of increased clay input. This relationship can be explained by carbonate precipitation by whitings. During episodes of high detrital input, photosynthesis activity decreases, which leads to a decrease in carbonate precipitation and δ 13C values. This detrital input appears to be controlled by variations of rainfall on the continent.