8.2: Classroom Activity

Sediment Core Analysis of the Ahramat Branch

Excerpt (Ghoneim et al.): The geophysical profile interpretation for the Ahramat Branch at this locality was validated using two sediment cores of depths 20 m (Core A) and 13 m (Core B) (Fig. \(\PageIndex{1}\)). In Core A between the center and left bank of the former branch we found brown sandy mud at the floodplain surface and down to ~2.7 m with some limestone and chert fragments, a reddish sandy mud layer with gravel and handmade material inclusions at ~2.8 m, a gray sandy mud layer from ~3–5.8 m, another reddish sandy mud layer with gravel and freshwater mussel shells at ~6 m, black sandy mud from ~6–8 m, and sandy silt grading into clean, well-sorted medium sand dominated the profile from ~8 to >13 m. In Core B on the right bank of the former branch we found recently deposited brown sandy mud at the floodplain surface and down to ~1.5 m, alternating brown and gray layers of silty and sandy mud down to ~4 m (some reddish layers with gravel and handmade material inclusions), a black sandy mud layer from ~4–4.9 m, and another reddish sandy mud layer with gravel and freshwater mussel shells at ~5 m, before clean, well-sorted medium sand dominated the profile from 5 to >20 m.

Sediments from the Ahramat Branch riverbed, which were collected from the two deep soil cores (cores A and B), show an abrupt shift from well-sorted medium sands at depth to overlying finer materials with layers including gravel, shell, and handmade materials. This indicates a step-change from a relatively consistent higher-energy depositional regime to a generally lower-energy depositional regime with periodic flash floods at these sites. So, the Ahramat Branch in this region carried and deposited well-sorted medium sand during its last active phase, and over time became inactive, infilling with sand and mud until an abrupt change led the (by then) shallow depression fill with finer distal floodplain sediment (possibly in a wetland) that was utilized by people and experienced periodic flash flooding. Validation of the paleo-channel position and sediment type using these cores shows that the Ahramat Branch has similar morphological features and an upward-fining depositional sequence as that reported near Giza, where two cores were previously used to reconstruct late Holocene Nile floodplain paleo-environments (Sheisha et al., 2022).

Figure \(\PageIndex{1}\): Deep sediment cores from the southern segment of the Ahramat branch. (From Ghoneim et al 2024.)

1. What are the lengths of Core A and Core B? What was the depth of the Ahramat branch as the paper by Ghoneim et al. has determined? Do these sediment core data tell us about the depth of the Ahramat branch at the time when the pyramids were built?
2. The excerpt says that the sediment cores show an abrupt shift from "well-sorted medium sands" at depth to overlying "finer materials". Where do this shift occur in Core A and Core B?
3. What do the core sediments containing "limestone, chert, and calcareous shale" tell us about the origins of the sediments? Terrestrial or aquatic?
4. What do the core sediments containing "unconformity and flooding" tell us about the nature of the sedimentation? High energy or low energy?
5. Assuming a deeper core sediment extractions than Core A and Core B were performed. At what depth would you see the bottom of the Ahramat branch based on the finding from other method in this paper? What types of sediment would likely be populated at that depth? Make a sketch of these hypothetical results.