Integrated sedimentology, mineralogy, geochemistry, and microfossil and
macrofossil biostratigraphies of the Maastrichtianearly Paleocene Dakhla
Formation of the Western Desert, Egypt, provide improved age resolution,
information on the cyclic nature of sediment deposition, and the
reconstruction of depositional environments. Age control based on
integrated biostratigraphies of planktic foraminifera, calcareous
nannofossils and macrofossils yields the following ages for
stratigraphic and lithologic sequences. The contact between the Duwi and
Dakhla formations marks the Campanian/Maastrichtian boundary (zone
CF8a/b boundary) and is dated at about 71 Ma. The age of the Dakhla
Formation is estimated to span from 71 Ma at the base to about 63 Ma at
the top (zones CF8a-Plc). The Cretaceous/Tertiary (K /T) boundary is
within the upper unit of the Kharga Shale Member and marked by a hiatus
that spans from 64.5 Ma in the lower Paleocene (base Plc) to at least
65.5 Ma (base CF2, base M. prinsii zones) in the upper Maastrichtian at
Gebel Gifata, the type locality of the Dakhla Formation. As a result,
the Bit Abu Minqar horizon, deposited between about 64.2 and 64.5 Ma
(Plc(l) zone), directly overlies the K/T boundary hiatus. Major hiatuses
also span the late Maastrichtian-early Paleocene in sections to the
northwest (c. 61.2-65.5 Ma at North El Qasr, c. 61.2-69 Ma at Bit Abu
Minqar and c. 61.2-65.5 Ma at Farafra), and reflect increased tectonic
activity. During the Maastrichtian-early Paleocene a shallow sea covered
the Western Desert of Egypt and the clastic sediment source was derived
primarily from tectonic activity of the Gilf El Kebir spur to the
southwest of Dakhla and the Bahariya arch. Uplift in the region resulted
in major hiatuses in the late Maastrichtian-early Paleocene with
increased erosion to the southwest. The area was located near the
palaeoequator and experienced warm, wet, tropical to subtropical
conditions characterized by low seasonality contrasts and predominantly
chemical weathering (high kaotinite and smectite). A change towards
perennially more humid conditions with enhanced runoff (increased
kaolinite) occurred towards the end of the Maastrichtian and in the
early Paleocene with shallow seas fringed by Nypa palm mangroves.
Sediment deposition was predominantly cyclic, consisting of alternating
sandstone/shale cycles with unfossiliferous shales deposited during
sea-level highstands in inner neritic to lagoonal environments
characterized by euryhaline, dysaerobic or low oxygen conditions,
Fossiliferous calcareous sandstone layers were deposited in
well-oxygenated shallow waters during sea-level lowstand periods. (C)
2001 Academic Press