60th Annual Report on Research 2015

The team has made significant headway in developing new datasets towards the project goals since the arrival of ACS-PRF funds. Notably, the age model for the Malawi 1B drillcore has not yet been published. Preliminary data that has appeared in conference abstracts indicates that the core is ~1.3 Ma, and it is likely that the geochronology that will be published will argue for linear sedimentation rates and depositional continuity throughout the Quaternary. Thus, there will be an excellent opportunity to use the detailed sequence stratigraphic interpretation from our PRF project as a rigorous framework with which the hypothesis of continuous sediment accumulation in a tectonically active lacustrine rift basin can be considered. Our lithofacies analysis indicates that several different depositional processes have influenced the 1B core site (including suspension settling, gravity flows) through time. Moreover, our early results suggest the strong likelihood for a transition in lake basin type (Carroll and Bohacs, 1999, Geology), based on variability in vertical parasequence stacking. Our work to date has identified a potential xenoconformity (Carroll, 2014, GSA Annual Meeting), which we believe marks an important change in Lake Malawi’s watershed history and provides direct evidence for tectonic influences on stratal development. Although no publications have been produced by the project team in 2015, the integrated stratigraphic results (including litho-, bio-, and chemofacies data) will be presented at the Geological Society of London Petroleum Group workshop, East Africa, From Research to Reserves in early 2016.

To date, the team has analyzed ~253m of drillcore at the scale of bedding, in order to build a new and highly detailed lithofacies log for the Malawi 1B core. This data is critical for examining facies dynamics and identifying important stacking patterns and chronostratigraphic surfaces. The work was completed using high-resolution core photos and x-radiographs obtained from the National Lacustrine Core Laboratory. The team collected 80 initial sediment sub-samples to initiate inorganic geochemical analyses, which have been completed using wavelength dispersive x-ray fluorescence (XRF) and instrumental neutron activation analysis (INAA). The XRF data have been useful for assessing trends in sediment composition and weathering patterns, especially through calculation of chemical index of alteration (CIA) values. The preliminary CIA dataset indicates that weathering patterns are variable over the length of the drill core, which we attribute to both climatic and tectonic changes that occurred in southeast Africa over the Quaternary. The INAA trace element data, generated at the USGS, are novel for muds from an extant East African rift lake, and our initial focus has been to scrutinize spider diagrams of the rare earth elements (REEs), both to understand the basic enrichment patterns and to assess if these data hold value for discerning variability in provenance or paleoenvironments in the basin. The team has compiled other available REE data from mudrock standards, as well as published data from the Eocene Green River Formation (western USA), one of the very few lacustrine deposits for which accessible REE data exist. The early results suggest that muds from Lake Malawi are strongly enriched in light REEs compared to the other mudrocks available for study, and cerium concentrations vary with depth, which is consistent with changing levels of lake-floor oxygen through time. The project team has also begun to examine trace element ratios, including Zr/Cr and Th/Sc, in order to test the sensitivity of these indicators in a lacustrine basin, and in an effort to trace changes in Lake Malawis sediment-generating hinterland through time, which is implied by the xenoconformity in the strata.

The project team has also begun developing organic geochemical datasets. Organic petrography (including UV and fluorescence microscopy) has been completed on a subset of the 1B sediment samples, and the results confirm that only immature organic matter is present. Vitrinite macerals are small and tend to be dispersed, perhaps due to the distal position of the 1B drill site with respect to the lake margin. Nonetheless, random vitrinite reflectance measurements produced a range of 0.29-0.35%. The team submitted six organic-rich sediment samples to the Applied Petroleum Geochemistry Laboratory at the University of Houston for kerogen isolation, which is the required feedstock for the proposed gold sealed-tube pyrolysis experiments. Kerogens were returned to us in August, and parameter testing is presently underway for the initial petroleum generation experiments. Significant effort has been directed towards the experimental setup and developing adequate gold tubing, such that the failure rate for individual experiments is minimized. The project team is also generating new total organic carbon and Rock Eval pyrolysis measurements using USGS facilities on the 80 Lake Malawi sediment samples, and initial results (highly variable hydrogen index values) indicate the need for a better understanding of mudrock diagenesis in the basin. Therefore, we are now also including fourier transform infrared spectroscopy (FTIR) measurements as part of our standard protocol for geologically young sediments associated with this project. The FTIR produces characteristic absorption spectra which correlate with the presence of mineral phases and organic materials. The Lake Malawi mudrocks tend to show strong absorbance peaks at 3720-3520 cm^-1 (clay minerals), 3000-2800 cm^-1 (aliphatic compounds) and 1540-1280 cm^-1(carbonate). The team is working to use these data, in concert with Rock Eval pyrolysis, to infer diagenetic shifts in organic properties over the length of the drillcore. The team plans to present the findings from this aspect of the research at the AAPG-SEPM Shale Diagenesis Research Conference in Santa Fe in October, 2016.

The second project year will witness another sample request, additional geochemical analyses, the completion of the stratigraphy and hydrocarbon generation experiments, as well as integration of all data into a 1D basin model. The impact of the PRF award has been substantial, as it has provided an important early-career mentoring opportunity for the PI, since three UK students have been directly supported by project funds. These students have learned key skills in the development and interpretation of sedimentological and geochemical data from scientific drill cores. The award has also been important for the development and marketing of a new petroleum geosystems educational program at UK.