Tuff fingerprinting and correlations between OGCP cores and outcrops for Pre-Bed I and Beds I/II at Olduvai Gorge, Tanzania
Introduction
The palaeoanthropologically-important Pleistocene strata of Olduvai Gorge, Tanzania contain abundant pyroclastic material derived from the adjacent Ngorongoro Volcanic Highlands, preserved in primary and reworked tuffs and as a component of various other volcaniclastic deposits (e.g., diamictites, conglomerates, sandstones derived from volcanic materials). A tephrostratigraphic framework based on these volcanic units helps correlate between different sites within the Olduvai Basin. Recent cores recovered by the Olduvai Gorge Coring Project (OGCP) from the Olduvai paleolake depocenter (Fig. 1) contain many of the same units, hosting a high-resolution paleoclimatic record. This study aims to characterize the tuffs and volcaniclastic sandstones of the OGCP cores in terms of their stratigraphic position, mineral assemblages, and mineral and glass compositions, and compare these against the mineralogical and geochemical “fingerprints” previously established for the Olduvai tuffs in outcrop. This work is therefore essential to establish correlations between the cores and the Olduvai outcrop stratigraphy, thus relating the paleoclimatic record derived from the cores to the classic Olduvai hominin sites. These sites document changes in hominin species (Homo habilis, Homo erectus, Paranthopus boisei, and Homo sapiens) and stone tool technologies (Oldowan, Acheulean, and Middle Stone Age), together with a record of the paleoecological context (e.g. Leakey, 1951, Leakey, 1965, Leakey, 1966, Leakey, 1971; Leakey et al., 1964). This study also documents the changes in volcanic compositions over time, which tracks the evolution of the Ngorongoro Volcanic Highlands through the Pleistocene.
Section snippets
Background
Tuffaceous units are used at Olduvai as chronostratigraphic time-planes throughout the Olduvai Basin, in some cases marking the boundaries between various Beds of the Olduvai Beds (e.g. Tuff IF at the boundary between Beds I and II). Such correlations have been based on physical mapping (e.g. Hay, 1976), and more recently based on mineral and glass compositional “fingerprints” (e.g. McHenry, 2005, McHenry, 2012; McHenry et al., 2016; Habermann et al., 2016; McHenry and Stanistreet, 2018). Fig. 2
Core sampling
Vertical boreholes OGCP-1A, 2A, and 3A were drilled at three different sites in the Olduvai Basin (Fig. 1) targeting the purported depocenters for the paleolake during different stratigraphic intervals, and a 23° inclined borehole was additionally drilled (3B) for paleomagnetic analysis. Cores were retrieved in 3 m intervals (6 cm core diameters), and the core segments were then cut into ~150 cm increments. For example, for the 15th interval collected for core 2A, the core would be broken down
Results
The results comprise major and minor element compositions for all major phenocrysts present within each sample, where possible from 15 to 17 grains or shards of each. Minor or non-target phenocrysts may be represented by a smaller number of grains analyzed, or a simple observation of presence/absence (thus the EPMA analysis was used only to confirm the identification of the phase). Table 1 tabulates the presence and qualitative relative abundances of each phase. Notes on the mineral
Proposed correlations
The EPMA results for the target phases analyzed for each sample were plotted against a database of published and unpublished analyses of tuffs from Olduvai outcrops (McHenry, 2005, McHenry, 2012; McHenry et al., 2016, Habermann et al., 2016); McHenry and Stanistreet, 2018), and against other tuffs from the same general interval within the cores, to look for similarities. Such similarities, or distinctive properties, were noted for each phase in each sample. Correlations are proposed for when:
Discussion
Fig. 5 shows the proposed tephra correlations between the three cores and a composite outcrop section, with the major tuffs indicated. Many tuffs recognized in outcrop do not appear to have equivalents in any of the cores.
While the Bed I tuff record from the OGCP cores match the outcrop record well (with all major marker beds identified in at least one core, and some marker beds identified in all cores), the correspondence between the cores and Bed II was disappointing. None of the major Bed II
Conclusions
In this study we identified all of the major Upper Bed I tuffs in one or more of the three Cores 1A, 2A, 3A, using mineral assemblages and compositions, allowing for direct correlations between the paleoenvironmental records in the cores and outcrops in the Olduvai Basin. No specific correlations can be made between Bed II tuffs in the cores and outcrops based on mineral assemblage and composition alone, due in part to the poor preservation of these tuffs in lacustrine sediments. New tuff
Declaration of competing interest
The authors avow that there are no conflicts of interest associated with this manuscript.
Acknowledgements
The authors would like to thank the Stone Age Institute for funding the Olduvai Gorge Coring Project (OGCP), with support from the Kaman Foundation, the Gordon and Ann Getty Foundation, the John Templeton Foundation, the Fred Maytag Foundation, Henry and Glenda Corning, and Kay and Frank Woods. Additional funding for the tephrostratigraphic work and core-to-outcrop correlations came from the National Science Foundation (BCS grant #1623884 to Njau and McHenry). We thank the Tanzanian Commission
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