Page 1 of 61 Sedimentology 1 Sedimentological and paleoenvironmental study from Waregi Hill in the Hiwegi Formation 2 (early Miocene) on Rusinga Island, Lake Victoria, Kenya 3 4 Lauren A. Michel1, Thomas Lehmann2, Kieran P. McNulty3, Steven G. Driese4, Holly Dunsworth5, 5 David L. Fox6, William E. H. Harcourt-Smith7,8, Kirsten Jenkins9 and Daniel J. Peppe4 6 7 1Department of Earth Sciences, Tennessee Tech University, Cookeville, TN 38505, U.S.A. 8 2Department Messel Research and Mammalogy, Senckenberg Research Institute and Natural 9 History Museum Frankfurt, Germany 10 3Department of Anthropology, University of Minnesota, Minneapolis, MN 55455, U.S.A 11 4 Terrestrial Paleoclimatology Research Group, Department of Geosciences, Baylor University, 12 Waco, TX 76798, U.S.A. 13 5Department of Sociology and Anthropology, University of Rhode Island, Kingston, RI 02881, 14 U.S.A. 15 6Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455, U.S.A. 16 7Department of Anthropology, Lehman College CUNY, Bronx, NY 10468, U.S.A. 17 8Division of Paleontology, The American Museum of Natural History, NY, NY 10024, U.S.A. 18 9Department of Social Sciences, Tacoma Community College, Tacoma, WA 98466, U.S.A. 19 20 Key words: paleosols, Ekembo, catarrhine evolution, sedimentology, paleoenvironmental 21 reconstructions Sedimentology Page 2 of 61 22 Abstract 23 Paleontological deposits on Rusinga Island, Lake Victoria, Kenya, provide a rich record of 24 floral and faunal evolution in the early Neogene of East Africa. Yet, despite a wealth of available 25 fossil material, previous paleoenvironmental reconstructions from Rusinga have resulted in 26 widely divergent results, ranging from closed forest to open woodland environments. Here, we 27 present a detailed study of the sedimentology and fauna of the early Miocene Hiwegi 28 Formation at Waregi Hill on Rusinga Island, Kenya. Our new sedimentological analyses 29 demonstrate that the Hiwegi Formation records an environmental transition from the bottom 30 to the top of the unit. Lower in the Hiwegi Formation, satin-spar calcite after gypsum in siltone 31 deposits are interpreted as evidence for open hypersaline lakes. Moving up-section, carbonate 32 deposits – interpreted previously as evidence of aridity – are actually diagenetic calcite 33 cements, which preserve root systems of trees; further up-section, the upper-most paleosol 34 layer contains abundant root traces and tree-stump casts, previously interpreted as evidence of 35 a closed-canopy forest. These environmental differences are reflected by differences in faunal 36 composition and abundance data from Hiwegi Formation fossils sites R1 and R3. Taken 37 together, this work suggests that divergent paleoenvironmental reconstructions in previous 38 studies likely suffered from time-averaging across multiple environments. Further, our results 39 demonstrate that during the early Miocene habitats in Rusinga’s Hiwegi Formation varied both 40 spatially and temporally. From a regional perspective, it has been argued that during the early 41 Neogene a broad forested environment stretched across the African continent, transitioning 42 later to predominately open landscapes that characterizes the region today. Our results 43 challenge this simple model, suggesting instead that local or regional habitat heterogeneity Page 3 of 61 Sedimentology 44 already existed in the early Miocene. This has important implications for interpretations of the 45 selective pressures faced by early Miocene fauna, including Rusinga Island’s well-preserved ape 46 and catarrhine primates. Sedimentology Page 4 of 61 47 1. Introduction 48 The Paleogene-Neogene transition was a time when Africa underwent tectonic, climatic, 49 and biological changes that would eventually set up the modern ecosystems seen across East 50 Africa today (e.g., White, 1983; Burke & Gunnell, 2008; Feakins & DeMenocal, 2010; Jacobs et 51 al., 2010; Partridge, 2010; Wichura et al., 2015). During the time of the Oligocene-Miocene 52 boundary, the Afro-Arabian plate began to collide with the Eurasian plate creating the Alpine 53 Orogeny and simultaneously closing the western Tethys Sea which created new land 54 connections between Africa and Eurasia (e.g., Dercourt et al., 2000; Stampfli et al., 2002; 55 Golonka, 2004). Firmly stabilized by the Burdigalian (early Miocene), the connections between 56 Africa and Eurasia established migration routes that enabled numerous faunal dispersals into 57 and out of Africa (for instance Rögl, 1997, 1999; Sen, 2013). As a consequence, African faunas 58 experienced major reorganization: some previously diverse clades went extinct (e.g., numerous 59 hyrax genera, ptolemaids) whereas other groups (e.g., Proboscidea) disersed and thrived out of 60 Afro-Arabia (Kappelman et al., 2003). 61 It has been argued that there was a pan-African tropical lowland forest during the early 62 Miocene that covered Africa from west to east with little variation in broad-scale ecosystems 63 until the eventual split of East African and Guineo-Congolian rainforests beginning at 16.8 Ma 64 (e.g., Andrews and Van Couvering, 1975; Couvreur et al., 2008; Wichura et al., 2015). In 65 constrast, it has also been argued that this idea is too simplistic, and that vegetation across 66 Africa would have been more heterogeneous throughout the Cenozoic (e.g., Jacobs et al., 1999, 67 2010; Jacobs, 2004). There is evidence for significant environmental heterogeneity between 68 sites and through time, although the number of sites from the Paleogene-Neogene transition in Page 5 of 61 Sedimentology 69 Africa is small (see discussion in Kappelmann et al., 2003). For example, there are forests during 70 the Oligocene and Miocene in Ethiopia (Pan & Jacobs, 2009; Pan et al., 2012) and Kenya 71 (Chesters, 1957; Michel et al., 2014; Oginga, 2017), but also well-documented open 72 environments and wooded grasslands (i.e., Hamilton, 1968; Jacobs et al., 1999; Kappelman et 73 al., 2003; Lukens et al., 2017; Liutkus-Pierce et al., 2019). While these sites offer the ability to 74 reconstruct the environment in high-resolution, they often suffer from being temporally 75 limited. Alternatively, more continuous records such as those from marine sediment cores or 76 from compiled isotope values of pedogenic carbonate have contributed to the broad-scale 77 argument that tectonic and climate shifts through the Cenozoic resulted in a major transition 78 from more closed environments in the early Miocene to more open habitats leading up to, and 79 particularly after, the Mid-Miocene Climatic Optimum (e.g., Ségalen et al., 2007; Wichura et al., 80 2015; Uno et al., 2016; Polissar et al., 2019). Ultimately, the link between small-scale 81 environmental evidence and bigger picture ecological change is not well established. This 82 results in part because most paleontological sites sample individual habitats that record 83 restricted intervals of time – making it difficult to assess how quickly environmental changes 84 may have occurred or whether multiple ecosystems existed at any one site through time. 85 Dated to the Burdigalian stage, Rusinga has played an important role in understanding 86 early Neogene African paleoenvironments (e.g., Andrews & Van Couvering, 1975; Andrews et 87 al., 1979; Evans et al., 1981; Collinson et al., 2009) owing to its tremendous preservation of 88 plant and animal remains. More than 100 vertebrate species are now known from the Rusinga 89 fossil beds, as are a wide variety of remains from other fossil animals and plants (MacInnes, 90 1943; LeGros Clark & Leakey, 1950; Shackleton, 1951; Pickford, 1984; Walker et al., 1993; Sedimentology Page 6 of 61 91 Walker, 2007; Michel et al., 2014) dating between about 20-17 Ma (Peppe et al., 2011, 2017b). 92 The majority of previous research has focused on the highly fossiliferous Hiwegi Formation, and 93 in particular strata identified as the Fossil Bed Member, from which an estimated 80% of the 94 Rusinga fossil specimens are thought to have been derived (e.g., Van Couvering, 1972; Pickford, 95 1984). However, our recent work (see Michel et al., 2014; Peppe et al., 2017b) has shown that 96 the “Fossil Bed Member” is not contemporaneous from site to site, as originally postulated 97 (e.g., Andrews & Van Couvering, 1975; Andrews et al., 1979; Evans et al., 1981; Drake et al., 98 1988; Collinson et al., 2009). Further, we have also demonstrated that the majority of the 99 Hiwegi Formation strata are fossiliferous (e.g., Maxbauer et al., 2013; Michel et al., 2014, 2017, 100 Peppe et al., 2016). This would have been a confounding factor in previous paleoenvironmental 101 reconstructions (e.g., Evans et al, 1981; Pickford, 1984) of the entire Hiwegi Formation that 102 relied on pooled geological samples or mixed fossil assemblages based on the assumption that 103 the “Fossil Bed Member” represented a discrete time interval across Rusinga. For this reason, it 104 is important to revisit previous ideas about Rusinga’s environment in the early Miocene, using 105 detailed geologic constraints for the stratigraphic position and inferred age of the fossiliferous 106 intervals within the Hiwegi Formation (Michel et al., 2014, 2017; Peppe et al., 2017a; b). 107 Here we present new sedimentological data combined with paleosol morphology and 108 micromorphology to reconstruct the paleoenvironments represented by the Hiwegi Formation 109 (early Miocene) exposed on Waregi Hill, Rusinga Island. Our composite stratigraphic section 110 through the Hiwegi