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Research Articles: Carbonate Sedimentology
The delineation of sequence stratigraphic patterns in deeper ramp carbonates is notoriously difficult using sedimentological criteria. Hence, in an effort to better understand the processes controlling the formation of deeper-water carbonate ramps, fully quantitative palynofacies analyses were carried out on two cores from the Upper Jurassic (Kimmeridgian) of the Swabian Marl Basin, SW Germany. To eliminate the effect of variable carbonate content, absolute palynoclast abundances (i.e., particles per gram of sediment) were calculated with respect to the non-carbonate fraction of the original sediment. The palynofacies data were supplemented by bulk sediment oxygen isotope data and sedimentological information.
Terrestrial and marine palynoclast groups show in-phase, cyclical absolute abundance patterns in both cores investigated. Highest carbonate-adjusted absolute abundances of terrigenous palynoclasts are interpreted to indicate sea-level lowstands and vice versa. The in-phase distribution of marine and terrestrial palynoclast groups points towards a coupling between the terrestrial and marine realms. Increased absolute abundances of terrigenous palynoclasts are interpreted to represent intervals of enhanced land-derived nutrient input into the marine system. Such higher nutrient availability would have caused higher phytoplankton productivity as evidenced in increased absolute abundances of marine palynomorphs. Hence, marine productivity in the epeiric setting of the Swabian Marl Basin was strongly controlled by the nutrient flux from the surrounding landmasses.
In the palynofacies and bulk sediment δ18O records, two hierarchies of sequences are recognized. Well-pronounced medium-scale sequences (with thicknesses of 5–20 m) are built by the stacking of smaller-scale sequences (with thicknesses of 3–7 m). Heavier δ18O values coincide with high absolute abundances of terrigenous palynoclasts. Based on this observation, a model for environmental fluctuations is proposed. Time intervals with high sea level were characterized by relatively high water temperatures, whereas time intervals with low sea level coincided with relatively low water temperatures. Estimates of ammonite zone ages available from the literature were used to constrain the duration of medium-scale sequences. When taking into account the range of dating uncertainties, the obtained value of ca. 370 kyr appears reasonably close to the duration of the 400 kyr eccentricity cycle. We speculate that the formation of medium-scale sequences as evidenced in our palynoclast and δ18O records may be related to eccentricity-driven insolation changes that caused sea-level changes, with higher insolation leading to a sea-level rise. This would imply that the regional record of sea-level and temperature fluctuations within the epeiric Swabian Marl Basin reflects climatic changes that occurred on a more global scale. The integration with sedimentological data shows that marl-dominated intervals were deposited during early sea-level rise and under cooler conditions, whereas limestone-dominated intervals formed during intervals of high sea level and under warmer conditions.
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