Sedimentary Systems Research group alum Neal Auchter (Ph.D. 2016) has a new paper out in Geology based on one of his dissertation chapters.
In this study, we use strontium isotope stratigraphy on reworked fossil fragments from the Cretaceous Tres Pasos Formation (southern Chile) to document sediment recycling. Strontium isotope stratigraphy is an established method to determine numerical ages of primary calcium carbonate from the 87Sr/86Sr ratio. This works better in some periods of Earth history than others, and the Late Cretaceous is one of those periods. Check out this 2012 review by McArthur and co-authors to learn all about this method.
However, we apply strontium isotope stratigraphy in a different way. Instead of determining 87Sr/86Sr from shelly fossils found in situ (e.g., in growth position), we made this measurement on fossil fragments found in coarse-grained turbidite deposits. In other words, the fossils were eroded, transported, and deposited some time after they formed. The conceptual diagram below (Fig. 1 from the paper) puts into context our use of strontium isotope ages with commonly used zircon (U-Pb) dating methods. The important difference between detrital zircon and detrital strontium, however, is that the latter helps constrain recycling from within the marine basin.
Over the course of a few field seasons, Neal and others in the Chile Slope Systems team collected shell-fragment samples from across the ~100 km-long Tres Pasos outcrop belt. The figure below (Fig. 3B from the paper) summarizes the detrital strontium isotope stratigraphy data (n=94 ages). The samples are organized on the horizontal axis by sample sets (L1-L13), which represent stratigraphic units whose depositional age generally youngs southward (to the right). The gray rectangles are maximum depositional age ranges as defined by our published detrital zircon work. The strontium ages are color/symbol-coded by specimen type (inoceramid, oyster, gastropod, and bivalve).
The primary observation in the data plot above is that very few of the strontium ages are within the gray rectangles. In other words, most of the strontium ages are older than depositional age and, in some cases, >10 Myr older. We interpret these fossil fragments to have been recycled during Tres Pasos deposition. For example, downcutting by a submarine canyon could have tapped into the underlying older deposits that were then transported further into the basin via turbidity currents. Considering the tectonically active setting, even minor uplift along the basin margin, could have contributed to the exhumation of shallowly buried deposits then available to be reworked further into the basin.
Although there is uncertainty about the exact mechanism (or combination of mechanisms) that led to the recycling we emphasize that with this method we can (1) document that intrbasinal recycling is occurring and (2) constrain the residence time of shelly fossils between initial calcium carbonate precipitation and eventual deposition. The figure below (Fig. 4 from the paper) depicts the regional stratigraphy and our interpretation of how proportions of specimen type (namely, inoceramid versus oyster) represent intrabasinal source terranes. Check out the paper — it’s open access! — for all the details.
Congrats to Neal on getting this work out there! It can take a lot of effort to get a thesis/dissertation chapter refined, submitted, and eventually published after graduate school is officially done (for example, when life events and new opportunities become the priority).