Nahid D. Gani

Blue Nile incision on the Ethiopian Plateau: Pulsed plateau growth, Pliocene uplift, and hominin evolution

The 1.6-km-deep Gorge of the Nile, a rival of the Grand Canyon, resulted from the deep incision of the Blue Nile drainage into the uplifted Ethiopian Plateau. Understanding the incision history of the plateau is crucial to unraveling the Cenozoic tectonoclimatic evolution of the region, particularly because the region has long been used as a natural laboratory to understand the geodynamics of continental rifting and the evolution of hominins. We undertake a quantitative geomorphologic approach integrating field, geographic information system (GIS), and digital elevation model (DEM) data to analyze incision (volume, long-term rates, and spatiotemporal variability) and river longitudinal profiles of the Blue Nile drainage. Previously published isotopic ages of the Cenozoic volcanic rocks are used to constrain long-term incision rates through geologic time. Our data argue that (1) the Blue Nile drainage has removed at least 93,200 km 3 of rocks from the northwestern Ethiopian Plateau since ca. 29 Ma (early Oligocene) through a three-phase (ca. 29–10 Ma, ca. 10–6 Ma, and ca. 6 Ma to present) incision, where long-term incision rates increased rapidly and episodically in the late Miocene (ca. 10 Ma and ca. 6 Ma); (2) being out-ofphase with the past climatic events and in-phase with the main volcanic episodes of the region, this episodic increase of incision rate is suggestive of episodic growth of the plateau; (3) of the ~2-km rock uplift of the plateau since ca. 30 Ma, 0.3 km was due to isostatic uplift related to erosional unloading, and the rest was due to other tectonic activities; (4) the extremely rapid long-term incision rate increase, thus a rapid uplift of the plateau, ca. 6 Ma might be related to lithospheric foundering, caused by ponded plume material beneath the Ethiopian Plateau and aided by huge tectonic stresses related to the Messinian salinity crisis of the Mediterranean Sea. These events could have caused the plateau to rise >1 km within a few m.y. in the early Pliocene. This uplift history of the Ethiopian Plateau can shed critical light on the geodynamics of the Afar mantle plume and the evolution of the East African hominins via climate change.

Creating three-dimensional channel bodies in LiDAR-integrated outcrop characterization: A new approach for improved stratigraphic analysis

In light detection and ranging (LiDAR)–integrated outcrop characterizations, coupled utilization of LiDAR-generated virtual outcrop models and the ArcGIS platform has been rarely pursued. As a consequence, there exists a limited appreciation of this coupled approach in stratigraphic analysis. This study presents a novel approach of three-dimensional (3-D) mapping of fluvial channel sand bodies in the Cretaceous Blackhawk Formation outcrops in Utah by exporting quantitative information from a high-resolution (∼10 cm) virtual outcrop model into ArcGIS. The adjoining and near-circular character of six contiguous cliff faces in our virtual outcrop model provided both upstream and downstream data sets, allowing us to gather adequate spatial data points ( x , y , and z coordinates for each point) for both basal and top bounding surfaces of individual channel sand bodies. For each sand body, these data points were manipulated in ArcGIS to generate a 3-D geobody, which is a realistic reconstruction of the stratigraphic preservation of that channel sand body in a sedimentary basin. The high resolution of our data set allowed the creation of this 3-D channel body down to individual channel-story level (single-story vs. multilateral). By creating and then populating all channel sand bodies of the entire Blackhawk Formation for our studied outcrop window, this technique generates a robust set of results that is useful for improved understanding of fluvial sand-body organization at a range of spatial scales, over both the short- (single to tens of thousands of years) and long-term (hundreds of thousands to millions of years). Our results are also important for improved reservoir and aquifer exploration and production strategy.

On the Environment of Aramis: Concerning Reply of White to Cerling et al. in August 2014

White (2014) made several comments on our paper (Gani and Gani 2011) while replying to Cerling et al. (2014). His comments contradict our field descriptions presented in Gani and Gani (2011). Based on our fieldwork at the Aramis site, Ethiopia, where a near-complete skeleton of Ardipithecus ramidus was excavated, we presented our field data with their analysis and interpretation in Gani and Gani (2011). We interpreted our data to indicate the presence of rivers and associated mixed vegetation (grasses and trees) in adjacent floodplains, which constitute the environmental context of the very place and time where/whenA. ramidus likely lived and died. It is a well-known fact that scientists can have different interpretations while scrutinizing the same data set, but their field descriptions (that constitute the data set) should not be contradicting. Therefore, in this reply, we focus on the basic field observations and descriptions (rather than the interpretations) of Gani and Gani (2011) that White questioned. White (2014) states that Gani and Gani (2011) “mistook a nearby tufa with uncemented mudstone pellets deposited in standing shallow water as a crossbedded fluvial sandstone” (471). Tufa is a nonclastic limestone formed by the in situ precipitation of carbonate minerals in fluvial or lake environments, whereas sandstone is a clastic sedimentary rock composed of sand-sized (0.063–2 mm) detrital (i.e., transported) grains like quartz, feldspar, and lithoclast. Therefore, it should not be difficult to differentiate tufa from sandstone in the field. Although we cannot discredit the presence of tufa in nearby localities, we observed the sedimentary rock in question with a hand lens (20#) and a grain-size card and determined it to be a fine-grained (0.125–0.25 mm) sandstone composed of various detrital grains including quartz. In these sandstone beds, we found numerous scattered lithoclasts of igneous origin that