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Figure 4. (A) Map showing distribution of average relative percentages of clay submergence by eustatic sea-level rise and
in Holocene sections and average rates of compaction (ARC) in mm/yr across land subsidence triggered by neotectonics
three northern delta sectors (I, II, III); dots indicate sites of 85 sampled drill and/or sediment compaction as discussed
cores (after Stanley and Clemente, 2014). (B) X-radiographs of selected drill above. This balance no longer occurs suf-
cores sections (locations shown in A): horizontal laminations in S-47; fault- ficiently at the Nile delta’s coastal sector
like shears(s) in S-31 and S-59; and convoluted strata units in S-18 and S-59. where amounts of discharged water and
Scale = 2 cm. (C) Location of seven submerged sites with archaeological sediment have diminished markedly in
material; the 11 numbers indicate depths (in m) of materials below present recent time due to altered climatic condi-
sea level (after Stanley and Toscano, 2009). (D) Plotted age and depth of sub- tions and much increased human impacts.
merged materials in C show all lie well below the Eastern Mediterranean Without addition of much needed super-
sea-level curve of Sivan et al. (2001), recording continued margin subsidence. posed deposits at the coast, the youngest
(earlier than 1000 A.D.), uppermost
percentages of clay are highest (40%–60%) layers generally occur between horizontal water-saturated sediment layers are now
in the N and NE sectors (II, III) where high strata above and below them, they likely being lowered at a more rapid rate relative
ARC values range from 7.7 to 8.4 mm/yr record the effects of natural episodic dis- to sea level (Figs. 4C and 4D).
(Fig. 4A). This finer-grained sediment turbance events. Some may have formed
fraction is preferentially displaced by cur- by rapidly increased sediment accumula- The Nile’s present total flow is contrib-
rents toward the NE delta margin (arrows tion and overloading. Others perhaps uted by relatively small, isolated areas in
in Fig. 5A). It is likely that water-saturated, resulted from recent earthquake tremors the East African lake region and the
clay-rich sediments readily expulsed much and ground motion, or from tectonic shifts Ethiopian highlands. The high precipita-
of their interstitial pore water shortly after and reorganization of underlying strata at tion at the headland of the White Nile is
burial by successive deposits and evapora- depth, as cited in the previous section. distributed between two rainfall seasons.
tion. In X-radiographs of Holocene delta Similarly, human-triggered effects also Water of the White Nile enters the Sudd
core sections, most displaying well- occur in areas where delta surfaces have marshes and seasonally flooded areas to
defined horizontal bedding (Fig. 4B, sec- been artificially lowered to ~1 m or more the north, where evaporation greatly
tion S-47), there are examples of interbed- and that are affected locally by liquefaction exceeds rainfall; this results in an outflow
ded strata that have been extensively as a result of hydrocarbon and ground- from the wetlands that is only about half
disturbed. Some display convolute stratifi- water extraction. that of inflow (Sutcliffe and Parks, 1999).
cation (Fig. 4B, in sections S-18 and S-59), In marked contrast, the Ethiopian moun-
interpreted as having failed by liquefaction DECREASED SEDIMENT tains, with their high rainfall in a single
and upward expulsion of interstitial water. REPLENISHMENT season and steep topography, produce
Other disturbed strata record sharp, fault- larger runoffs and more concentrated
like shear offsets (Fig. 4B, arrows and An ample sediment supply provided flows in the Blue Nile and Atbara during
symbol S in S-31 and S-59). Because such regularly to a delta’s margin helps mini- shorter periods.
mize or moderate problems of coastal
During much of the Holocene, amounts
of sediment transported from upland Nile
sources and dispersed northward to the
delta margin have been largely controlled
by Nile hydrologic attributes responding to
major long-term climatic variations. Strong
rains prevailed, especially during the latest
Pleistocene-Holocene Wet Phase that, in
Ethiopia, lasted ~6500 yr, between ca.
9000 B.C. and ca. 2500 B.C. (Said, 1993,
his figure 2.12). The rain-front at that time
shifted northward 8–10 degrees in the Nile
Basin, over arid sectors of the Sahel and
sectors of the Sahara in the Sudan and
Egypt. Such rainfall patterns responded
largely to fluctuation of the Inter-Tropical
Convergence Zone (ITCZ) that induced
major Nilotic hydrologic and sedimenta-
tion changes due to low-latitude isolation
forcing over long periods. On a shorter
(sub-millennial) scale, Nile valley climatic
input by El Niño Southern Oscillations
(ENSO) were also influential (Said, 1993;
Marriner et al., 2012). During the African
humid phase, strong boreal summer insu-
lation produced higher rainfall in northern

8 GSA Today | May 2017