Peter A. Bentham’s scientific contributions

From its inception during the early Miocene, the Suez Rift has been dominated by marine sedimentation. New high-resolution biostratigraphic and sedimentologic analyses of synrift deposits have resulted in the recognition of late Burdigalian-early Langhian brackish water and lacustrine deposits in the Wadi Abu Gaada-Gebel Gushia area, Sinai Peninsula. The Abu Gaada section is unique because: (1) it is an anomalously thick section of non-calcareous shale and mudstone in the Lagia Member of the Ayun Musa Formation; and (2) the mudstones contain an abundant microflora consisting of marine and nonmarine diatoms and freshwater algae that indicate they were deposited in a freshwater to brackish water environment. The abundant freshwater and shallow marine algae include the nonmarine diatoms Aulacoseira Thwaites, Fragilaria construens (Ehrenberg) Grunow, Synedra ulna (Nitzsch) Ehrenberg, and SurrirellaTurpin, as well as the freshwater algae Botryococcus Kutzing and Pediastrum Meyen. Shallow marine diatoms are represented by Actinoptychus Ehrenberg, Actinocyclus ehrenbergii Ralfs, Paralia sulcata (Ehrenberg) Cleve, Rhaphoneis Ehrenberg, and HyalodiscusEhrenberg. Correlations with other coeval stratigraphic sections in the Sinai Peninsula indicate that brackish water and lacustrine deposition was localized in the Wadi Abu Gaada-Gebel Gushia area. This is explained by uplift and tilting of the bounding faults separating the Wadi Abu Gaada-Gebel Gushia block from adjacent rift blocks where normal marine conditions prevailed. The increased freshwater influence was the result of a rising water table during a period of rising sea level. As transgression continued, the barrier that separated the Wadi Abu Gaada depression from marine waters was eventually breached. Brackish water then filled the depression. Eventually a hypersaline lagoon formed in what previously had been a fresh waterlake, depositing the Markha Anhydrite. Correlative brackish water/lacustrine shales also have been found in the subsurface of the Gulf of Suez and thin coeval deposits are present on its southwestern margin. This reflects that the local occurence of these depositional environments was controlled by structural reorganization of fault blocks in the Suez Rift in response to the “mid-clysmic” tectonic event.

Paleontological data derived from well cuttings and outcrop sections shed light on the chronostratigraphy and sequence stratigraphy of Miocene syn-rift rocks in the Gulf of Suez (GOS), and magnetostratigraphy of Miocene outcrop sections enhances the resolution of correlations. Graphic correlation of microfossil datums reveals that the Neogene section in the GOS consists of at least eight widespread biostratigraphic sequences (S10-S70) that are separated by major low order hiatuses or graphic terraces (T00-T60). In some places, such as atop old structural highs, hiatuses merge and the intervening sequence thins or disappears. Elsewhere, as in half graben lows adjacent to footwall blocks, hiatuses may be absent or not easily detectable. Higher order, minor hiatuses (and parasequences) within the Miocene section are undetectable by graphic correlation because of limitations inherent to the technique and lost resolution of ditch cuttings samples. Nevertheless, graphic correlation provides visual displays of time-stratigraphic relationships that are more accurate and useful than traditional biozonations. Such displays are also easily interpreted within the context of sequence stratigraphy (Fig. 1). Combining magnetostratigraphic techniques with graphic correlation has allowed us to significantly refine correlations and has provided additional absolute age control within the S10, S20, and S30 biostratigraphic sequences. Lateral magnetostratigraphic correlations were made successfully for a number of surface outcrop sections spanning the S10 and S20 sequences. Furthermore, we can then use these correlations to make estimates of the amount of time missing across the T10 and T20 biostratigraphic terraces. Field analysis in the Sinai of terraces T00 through T30 indicates that they represent either major regressive or transgressive events. T00 is a major unconformity (depositional sequence boundary) that separates the earliest syn-rift Miocene section from older pre-rift rocks. T05 is detected as a slight angular unconformity at Wadi Nukhul where it separates a dominantly continental red-bed section from overlying estuarine and shallow marine rocks. Magnetostratigraphic correlation of the Wadi Nukhul section suggests that T05 comprises only a brief period of geologic time. The T10 event, exposed at Wadi Thal, is associated with rapid subsidence of the GOS. The hiatal interval (40m) comprising T10 time at Wadi Thal has two ravinement surfaces (Glossifungites) and one shale-enclosed Trypanites hard ground. The hard ground may represent a period of sediment starvation within portions of the GOS at that time. T10 at Wadi Thal is therefore associated with an overall marine transgression and is comprised of at least three minor high order hiatal events represented by two ravinements and a condensed (hard ground) interval. These minor hiatuses cannot be resolved by graphic correlation as distinct events, but their sum total within a thin rock (hiatal) interval is detectable as a terrace. The magnetostratigraphic expression of the S10 interval was studied primarily within the Wadi Nukhul area. The lower red-bed section shows a series of normal and reverse polarity intervals.