Ian A. Brookes

Geomorphology and Quaternary geology of the Dakhla Oasis Region, Egypt

Abstract Dakhla Oasis (25.5°N, 29°E) occupies a structurally localized depression at 90–140 m above sea level, ∼1200 km 2 in area, below a 300 m escarpment bordering the Libyan Plateau, Western Desert of Egypt. Semi-arid intervals of the Quaternary period generated eight sedimentary formations, separated by erosion during hyper-arid intervals. Sediments comprise three generations of colluvial/fluvial fanglomerates, two generations of tabular spring-laid clastic and chemical sediments, two generations of mound springs and basinal sediments. of fluvio-lacustrine, evaporitic, pluvio-aeolian and aeolian origins. Discussion of these sediments is organized according to geomorphic region, from north to south, plateau, scarp and piedmont, lowland and cuesta plain. Chronological evidence is restricted to many radiocarbon dates of Holocene cultural material associated with playa sediments (9−4.5 ka), a Th U isochron age of ∼62.0 ±7.6 ka for basinal evaporites, and two Th U ages of ∼176 and ∼170 ka for a boulder of derived travertine. The regional Quaternary sequence is reconstructed from stratigraphic and geomorphic relationships of the sediments and erosion surfaces. It is broadly similar to sequences earlier reconstructed in the topographically similar Kharga Oasis region 150 km to the east, and Kurkur Oasis, 400 km to the south-southeast. Speculations on chronology and driving mechanism are offered in conclusion.

Aeolian erosional lineations in the Libyan Desert, Dakhla Region, Egypt

Abstract This paper presents results of a study of aeolian erosion at the landscape scale. There have been few such studies in desert geomorphology compared to those focused on individual landforms and on rock surface sculpture. The present study area lies in the southern part of the Libyan Desert in south-central Egypt, between 25° and 27°N, 29° and 30°E. Bedrock comprises Paleogene limestone of various lithologies. Climate is hyperarid. The following topics are treated. (i) Correspondences between rock outcrop belts and belts of terrain lineated by aeolian erosion and unlineated terrain. (ii) The effect of lithology on aeolian lineation, which works through the presence or absence of chert in the limestones. Lineation is produced by aeolian erosion in chert-free/poor rocks, whereas erosion of chert-rich rocks produces a desert pavement that armors the surface, suppressing erosion. (iii) The effect of large valleys eroded upwind of lineated terrain, which deflect winds and trap sand, ending erosion, so that downwind lineations are reduced and finally erased by weathering. (iv) Sample lineated landscapes in the area show stages of evolution, arranged in the time domain into a proposed cycle of aeolian erosion; the cycle progresses from initial smooth plain to grooves separating long, blade-shaped ridges, to segmentation of ridges into shorter blades along diagonal joints, to streamlining of shorter blades and size reduction, to final planation. In any one wind-parallel swath of lineated terrain, stages in the cycle progress downwind in the space domain, so that upwind landscapes are more advanced in the cycle. (v) The structure of air flow responsible for lineation is still uncertain, but initial grooving may respond to stable longitudinal horseshoe vortices, or self-organized regularity of erosion by random vortices; subsequently, the erosion pattern is fixed by evolving relief. (vi) Large, smooth basins within the lineated terrain were carved by aeolian erosion before Oxygen Isotope Stage (OIS) 5 (70–130 ka), as were similar basins along the Dakhla piedmont below the scarp; little geomorphic change has occurred since, so in this field of aeolian erosional lineations (AELs) the aeolian erosion cycle may have occupied 10 times as long. Since continental scale aridity set in at ca. 2.4 Ma, there have been only two cycles in this area. A completed cycle earlier than the current incomplete one is indicated by rare small yardangs riding ‘piggyback’ on larger ones.

Geomorphic indicators of Holocene winds in Egypt's Western Desert

Abstract Geomorphic mapping of Egypts Western Desert from LANDSAT-MSS images reveals oriented aeolian landforms that record, in part, Holocene winds. Wind directions reconstructed from these landforms indicate the dominance of N–S airflow from 30°N to 20°N, turning clockwise southward to NE–SW, conformable with modern circulation. A second direction appears over western Egypt, W between 30°N and 26°N, NW between 26°N and 20°N. Cross-cutting aeolian landforms show that W/NW winds are older than the N/NE winds. Geomorphic evidence, abundant south to 26°N and less abundant to 20°N, also indicates that W and NW winds were early Holocene ‘palaeowesterlies’. Some evidence also indicates that they extended eastward to at least 30°E, perhaps to the Red Sea. These winds steered moist Atlantic/Mediterranean air masses to Egypt, sustaining early Holocene lakes and playas north of the limit of tropical monsoonal rainfall at 20°N. Upon aridification, beginning after 5 kyr BP, yardangs oriented west to east were eroded in early Holocene basinal sediments in western Egypt, indicating that these winds continued there for 1–2 kyr, until 3–4 kyr BP. Optically stimulated luminescence (OSL) ages of surface sand sheet in southern Egypt indicate that the present north–south winds were established ca. 3–4 kyr BP, at the same time as the northern savanna boundary was stabilized at its present position.

Early Holocene basinal sediments of the Dakhleh Oasis region, south central Egypt

Abstract Twenty samples of artifactual ostrich eggshell and hearth charcoal, firmly to loosely associated with basinal lacustrine, playa, and sand sheet sediments in the Dakhleh Oasis region of south-central Egypt, yield radiocarbon ages between ca. 8800 and ca. 4700 yr B.P. The sediments record variable sedimentary responses to an early Holocene pluvial interval in this virtually rainless region. Differences of hydrogeology and morphometry among and within basin types complicate paleoclimatic interpretation.

Fossil Ice Wedge Casts in Western Newfoundland

The purpose of this note is to record the location and form of what are tentatively identified as fossil ice wedge casts in Western Newfoundland, and to comment on their origin, age, and palaeoclimatic implications. During geomorphological investigations in the area between Port aux Basques and Bonne Bay, continuing since 1963, these features have been identified in only two localities (Fig. 1). At St. Davids, one cast was located in 1963 but has since been destroyed by coastal cliff recession. At York Harbour, a new borrow gravel pit has exposed five casts.