Shallow-marine carbonate-dominated archives record intricate combinations of environmental, biological and diagenetic variables, enclosing valuable information. Competing sedimentary sources often result in complex facies... [ view full abstract ]
Shallow-marine carbonate-dominated archives record intricate combinations of environmental, biological and diagenetic variables, enclosing valuable information. Competing sedimentary sources often result in complex facies successions that may in turn have different susceptibilities to post-depositional processes. Such records are therefore challenging for paleoenvironmental studies, and even more so dealing with elemental chemostratigraphy, an intrinsically complex approach. A representative example of such intricate records is here explored, the São Julião section, located in the southern part of the Lusitanian Basin (Ericeira, Portugal). This section comprises alternating marly, carbonate and sandstone-rich coastal-marine deposits of Early Albian to Early Cenomanian age, representing fluctuations between terrestrial and shallow-marine depositional environments. This site is therefore ideal for testing the validity of statistic tools for interpreting trace elemental records from ancient shallow-marine deposits.
At first sight, obtained major and trace elements (Ca, Mg, Sr, Fe and Mn) revealed a rather chaotic stratigraphic record. But when treated by a two-step principal component analysis, four major elemental associations were identified. These can be related to lithology: 1) very high content of Mg and Fe, interpreted to reflect the presence of dolomite; 2) very low Ca and Sr concentration, represented by samples from sandstone beds; 3) higher Fe and Mn content, in response to a higher siliciclastic contribution; 4) higher Ca and Sr values corresponding to more massive limestone horizons. Field and petrographic observations, as well as other independent proxies (X-ray diffraction and energy-dispersive X-ray spectroscopy) validated this approach, perfectly fitting the statistically obtained groups of samples. Standard elemental screening was performed to both calcite dominated groups of samples (groups 3 and 4), including samples with and without strong siliciclastic influence. Obtained relationships between tested elements did not correspond to well known diagenetic trends, so they may therefore be considered relevant for assessing paleoenvironmental conditions. In contrast, the presence of dolomite was probably related to later diagenetic processes. A stratigraphic representation of best preserved sample materials shows fluctuations in siliciclastic contribution from Late Albian towards Early Cenomanian times, suggesting marked paleoenvironmental changes during the studied time frame. This is in agreement with previous research, hereby further complemented.
Topics: Shelf and shallow water sedimentation , Topics: Carbonate platforms and reef
