Testing carbonate chemostratigraphy across differentiated ancient shallow-platform environments (Early Kimmeridgian, S Iberia)

摘要

Shallow-platform settings with marked differences in paleoplatform bottom physiography influence the degree of connection with oceanic waters and overall circulation patterns, even when sharing the same palaeoclimatic conditions. Two Kimmeridgian shallow-marine settings have been explored to test the sensitivity and reliability of carbonate chemostratigraphy to detect such differences. An integrated overview of the obtained elemental trends depicted four major facies, shared along specific stratigraphic intervals of both depositional records. Diagenesis obliterated original geochemical signals only throughout the siliciclastics-rich interval, corresponding to the most landward setting. For the remaining facies, elemental features could be attributed to the differential action of forcing mechanisms operating along the south-Iberian paleomargin during Kimmeridgian times. The highest degree of continental influence can be recognized by a strong relationship between Fe and Mn for the most proximal setting, which fades out along the mixed carbonate-fine siliciclastic rhythmic deposition in more open settings. A characteristic geochemical signature of progressively more positiveδ13C values and significantly higher Sr content is identified for the interval dominated by biogenic sponge buildups. Such a local response is related to local forcing by upwelling in the surroundings of a coral fringe. The geochemical signature of a hydrothermal origin can be clearly differentiated from the influence of mere terrigenous pulses. Accordingly, the decoupling of Fe and Mn along marginal settings is the clue to detecting major events of palaeogeographic restructuring. Observed temporal variations in Mg content along both studied sections are attributed to tectonic activity influencing nearshore/coastal water masses. By integrating chemostratigraphic information and complementary evidence, the palaeoenvironmental mechanisms promoting differentiated sedimentary records along ancient subtropical, shallow, coastal settings can be disentangled.