6 Integrating 20 Years of Scientific Drilling in the East African-Syrian Rift: A Session In Honor of Andrew Cohen, Part I

Continental drilling extending from the East African Rift into western Asia has yielded transformative discoveries, altering our understanding of paleoenvironments, paleoclimatology, paleoecology, and paleoanthropology. This session, in honor of Andrew Cohen, will celebrate these successes and define new directions.

Sunday, 19 October 2025
8:00 AM - 12:00 PM
HBGCC, 214A

Age Model and Depositional Environments of the Pliocene Northern Awash Cores of the Hominin Sites and Paleolakes Drilling Project (HSPDP)

Paper number: 6-3
Program: Technical Sessions
Time: 8:50 AM – 9:05 AM

Authors

Abstract

The Ethiopian Northern Awash (NA) site of the Hominin Sites and Paleolakes Drilling Project (HSPDP) was one of five targets in eastern Africa to generate continuous, high-resolution paleorecords from lacustrine sediments associated with key hominin fossil and archaeological sites. Three boreholes from two NA locations ~3 km apart (NAO and NAW) resulted in over 600 m of collected core and a composite thickness of ~285 m with ~98% recovery and significant overlap between boreholes. The cores are dominated by fine-grained sedimentary strata, primarily laminated or massive mudstone with rare sandstones, but also include three interbedded basalt flows from 10-65 m thick. The majority of the NA sequence ranges from ~2.9-3.2 Ma, equivalent to the Kada Hadar Member of the Hadar Formation. The lowest sedimentary package below the 65 m basalt is likely older than 3.4 Ma and the uppermost several meters are <2 Ma. Temporally equivalent strata indicate a sedimentation rate almost three times higher in the NA cores compared to the fossiliferous outcrops at Hadar ~30 km away. However, constructing a reliable stratigraphic age model for application to paleoenvironmental proxies has been hampered by several factors including: the limited number, and vertical distribution, of feldspar-bearing tephras suitable for 40Ar/39Ar analysis; the large uncertainties on basalt ages; the unsuccessful reorientation of sedimentary paleomagnetic samples; and the context of each basalt emplacement (subaerial vs. subaqueous) when factoring in basalt thickness. Facies and microfossil analysis of the cores indicate that deposition was lacustrine dominated, but with numerous intervals of subaerial exposure and pedogenesis. The NAO location was closer to the shoreline compared to NAW and five broad depositional phases have been identified. The lower part of the sequence begins with a deep anoxic lake accompanied by subaqueous basalt emplacement, transitioning to a stable, but shallower lacustrine environment. The upper portion includes multiple lacustrine episodes of variable depth, alternating with subaerial exposure represented by pedogenically modified mudstones and deeply weathered basalt flows. In situ brecciation, altered mudstones, sedimentary dykes, and reset (U-Th)/He dates indicate hydrothermal activity. The numerous transitions between lacustrine and terrestrial conditions recorded in the NA cores highlight the dynamic nature of the landscape Pliocene hominins traversed, whereas the temporally equivalent outcrops at Hadar record only four brief lacustrine transgressions.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-413240

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