Our research in sedimentary processes is focused on the Cenozoic, and includes work on tracing fault-basin fluid flow, lacustrine environments, and polar provenance as an insight into global climate change.
Fault-basin Fluid Flow
Geologists have long recognized the potential of faults in the upper crust to act as conduits for subsurface fluid flow, and the importance of such flow to petroleum geologists and hydrologists has spurred considerable research. However, basin scale, tectonic controls on fault zone architecture and permeability structure remain poorly understood. Work conducted by Professor Laurel Goodwin and Ph.D. candidate Randy Williams utilizes fault zone calcite cements as a geochemical record of fluid source in an effort to constrain the tectonic controls on fault zone fluid flow in the Rio Grande rift (Figure 1). This multi-disciplinary collaboration between members of the structural geology and ICP-TIMS laboratories examines the C, O, Sr, and clumped isotope content of calcite cements in fault zones to constrain the stratigraphic depth of fluid transport. Results of these analyses were recently published in Geology, and demonstrate that extension and syntectonic sedimentation result in a predictable spatial and temporal distribution of fault zone permeability structures, resulting in flow pathways which transmit fluids from different stratigraphic levels depending on slip magnitude and basin position. As the general pattern of sedimentation and faulting observed in the Rio Grande rift is similar to most other rift basins around the world, these results provide a fundamental first step toward accurate prediction of where and when fault zones will serve as conduits for fault-parallel transport in extensional tectonic environments. This research is ongoing, and future work is focusing on integrating geochronology data with fluid source studies in an effort to constrain the episodicity of deformation and fluid flow in seismogenic fault zones.
Lake systems are highly sensitive to climate and source changes as compared to the global marine system. In a major project led by Prof. Alan Carroll, we have been studying Sr isotope variations in he Green River Formation (Wyoming), which represents a major Eocene lake system in the western U.S., ancient Lake Gosiute. 87Sr/86Sr ratios are used as a novel geochemical tool to investigate chemostratigraphic correlation, lake level, water chemistry, and provenance and diagenesis history. 87Sr/86Sr ratios are not fractionated during precipitation of carbonate phases, nor are 87Sr/86Sr ratios systematically reset by diagenesis. Because, however, the Green River Formation was fed streams that traversed a diverse range of ages and bedrock lithologies, 87Sr/86Sr ratios are correlated with lake levels and depositional history. Our work has shown that lake expansion is correlated with low (non-radiogenic) 87Sr/86Sr ratios, whereas lake contraction is associated with more radiogenic 87Sr/86Sr ratios (Figure 1). Moreover, 87Sr/86Sr ratios can be used to correlate lacustrine facies at least up to 25 km apart. There is also evidence that groundwater might have contributed to the Lake Gosiute water budget during high stand.
This work is currently being spearheaded by Ph.D. student M’bark Baddouh.
We are conducting sediment provenance studies to evaluate the response of polar ice sheets during past climatic changes. The response of polar ice sheets to global warming is necessary in order to evaluate the amount of sea level rise that will occur as global temperatures increase. This work is being spear headed by Professor’s Anders Carlson and Joseph Stoner at Oregon State University. The work done at Wisconsin has been to collect and help interpret Sr-Nd-Pb isotope records from Greenland and to aid in the interpretation of Sr-Nd-Pb isotope records that will be collected from Antarctic sediments.
The general principle of this method is based on the fact that when ice sheets occupy their maximum position, they produce sediment via erosion of the substrate, which is discharged to the ocean as meltwater plumes and IRD if marine terminating (Figure 1). During ice sheet retreat, sediment is eroded by the high meltwater flux in proglacial streams and runoff from precipitation/snow melt (Figure 1). The major flux of sediment to the ocean occurs during deglaciation. This sediment is delivered to the ocean basin and through the use of the provenance indicators such as remnant magnetization and chemical and isotopic compositions it is possible to reconstruct the geologic terrane from which the sediment was derived. As a glacier retreats off of a geologic terrane the sediment flux from that terrane will decrease causing a change in the magnetic properties and chemical and isotope composition of the sediment that is transported to the ocean basin (Fig. 1). Importantly, our studies have concentrated on the silt fraction of sediment and avoids analysis of sand rich sediments. Sand rich sediments in the open oceans are derived from ice rafted debris. Additionally by concentrating on silt sized material we can avoid some of the ambiguities that can occur from far traveled sediment transported by boundary currents which are only able to transport significant amounts of clay sized material.
We have applied this provenance work on sediments deposited in the Eirik Drift off the coast of Greenland and collected in core MD99-2227 (Figure 2). The bedrock geology of Greenland is overwhelmingly Precambrian in age, and from south to north consists of the Ketilidian Mobile Belt (KMB), the Archean Block (AB), and the Nagssugtoqidian (NMB) Mobile Belt (Figure 2). During the Paleogene there was significant magmatism associated with the breakup of the north Atlantic producing a fourth tectonomagmatic terrane that is referred to as Paleogene Volcanics (PV; Figure 2). Each of these terranes have their own unique Sr-Nd-Pb isotope signature where the Precambrian age terranes have high 87Sr/86Sr and low eNd values as compared to the PV. The isotope composition of these different terranes have been measured based on the analysis of silt sized material from recent fluvial deposits from rivers that originated from these different terranes (Figure 2 shows sampling sites). Using these endmember compositions and simple mixing equations we are able to calculate the relative amounts of each terrane responsible for the isotopic composition of silt from core sediments in the Eriki drift (Colville et al., 2011 and Reyes et al., 2014). For example, during Marine Isotope Stage 11 (MIS 11), a time associated with substantial sea level rise (~7m) that took place between 424,000 and 374,00 years ago, Eirik drift core sediments have relatively high eNd values and low 87Sr/86Sr ratios as compared to core sediments of younger age. Such isotopic compositions can only be produced if the Precambrian Greenland terranes did not supply significant amounts of sediment. The lack of a Precambrian signature in MIS 11 sediments signifies that there was a minimal amount of ice sheet on southern Greenland. Indeed, modeling would suggest that nearly 6m of sea level rise during MIS 11 can be produced by loss of the Greenland Ice Sheet as shown by our provenance studies (Figure 2 and Reyes et al., 2014). This implies that the Antarctic Ice Sheet could have been relatively stable during MIS 11.
As a follow up to these studies conducted on Greenland we are preparing to begin similar provenance studies on the West Antarctic Ice sheet to evaluate the stability of the ice sheet during past interglaciations (Figure 3). Although we will use similar methods as we used for our studies of the Greenland Ice Sheet these are likely to be more challenging because the Sr-Nd-Pb isotope contrast in the bedrock underlying the west Antarctic Ice sheet are muted because these rocks are late Precambrian and Phanerozoic in age which will not produce the same amount of isotopic contrast in these different terranes as compared to the terranes in Greenland which were Archean to early Proterozoic in age.
Reyes, AV, Carlson, AE, Beard, BL, Hatfield, RG, Stoner, JS, Winsor, K, Welke, B, and Ullman, DJ (2014) South Greenland ice-sheet collapse during Marine Isotope Stage 11. Nature 510:525-528.
Colville, E.J., Carlson, A.E., Beard, B.L., Hatfield, R.G., Stoner, J.S., Reyes, A.V., Ullman, D.J. (2011) Sr-Nd-Pb Isotope Evidence for Ice-Sheet Presence on Southern Greenland during the Last Interglacial. Science 333:620-623.Add text