9 research outputs found
New excavations in the MNK Skull site, and the last appearance of the Oldowan and Homo habilis at Olduvai Gorge, Tanzania
MNK Skull is one of the most significant archaeological sites in Olduvai Gorge, particularly due to the previous discovery of human fossils referred to in the paper where the Homo habilis taxon was originally defined. An important archaeological assemblage is contained in the same horizon as the hominin fossils, constituting the last evidence of both Homo habilis remains and handaxe-free tool kits in the Olduvai Gorge sequence. Our excavations at the site are the first to be conducted since the original work in the 1960s, and sought to refine the archaeological context wherein the Homo habilis remains were discovered. Chronostratigraphic results place the MNK Skull sequence in Middle Bed II prior to deposition of Tuff IIB. The assemblage was deposited near the shoreline, as Palaeolake Olduvai withdrew into the basinal depocentre, and fossils and stone tools were subjected to significant post-depositional processes. The assemblage was affected by mudflow deposits that buried and preserved the assemblage but also entrained surficial bone and lithic elements into the flow. Rather than an occupation site as originally interpreted, the assemblage is better understood as a background deposit, possibly accumulated on an unconformity surface over a long period of time. The stone tool assemblage is typical of the Oldowan, with no technological elements announcing the appearance of the Acheulean, which is well attested to across the Olduvai sequence in post-Tuff IIB times. Our results highlight that, with an approximate age of circa 1.67 Ma, MNK Skull stands as a key site to understand the late Oldowan and the disappearance of Homo habilis in East Africa.Peer reviewe
The Kaapvaal Craton, South Africa: no evidence for a supercontinental affinity prior to 2.0 Ga?
We briefly examine the possible antiquity of the supercontinental cycle while noting
the likely unreliability of palaeomagnetic data >ca.1.8 Ga, assuming a gradual
change from a magmatically dominated Hadean Earth to a plate tectonically dominated
Neoarchaean system. A brief review of one of Earthâs oldest cratons, Kaapvaal,
where accent is placed on the lithostratigraphic and geodynamic-chronological history
of its cover rocks from ca. 3.1 to 2.05 Ga, forms the factual basis for this article. The
ca. 3.1â2.8 Ga WitwatersrandâPongola (Supergroups) complex retroarc flexural foreland
basin developed while growth and stabilization of the craton were still underway.
Accretion of relatively small composite granite-gneiss-greenstone terranes (island arc
complexes) from both north and west does not support the formation of a Neoarchaean
supercontinent, but may well have been related to a mantle plume which enhanced primary
gold sources in the accreted terranes and possibly controlled the timing and rate
of craton growth through plate convergent processes. Subsequent deformation of the
Witwatersrand Basin fill with concomitant loss of â€1.5 km of stratigraphy must have
been due to far-field tectonic effects, but no known mobile belt or even greenstone
belts can be related to this contractional event. At ca. 2714â2709 Ma, a large mantle
plume impinged beneath the thinned crust underlying theWitwatersrand Basin forming
thick, locally komatiitic flood basalts at the base of the Ventersdorp Supergroup, with
subsequent thermal doming leading to graben basins within which medial bimodal volcanics
and immature sediments accumulated. Finally (possibly at ca. 2.66â2.68 Ga),
thermal subsidence enabled the deposition of uppermost Ventersdorp sheet-like lavas
and sediments, with minor komatiites still present. Ongoing plume-related influences
are thus inferred, and an analogous cause is ascribed to a ca. 2.66â2.68 Ga dike swarm
to the north of the Ventersdorp, where associated rifting allowed formation of discrete
âprotobasinalâ depositories of the Transvaal (ca. 2.6â2.05 Ga Supergroup, preserved
in three basins). Thin fluvial sheet sandstones (Black Reef Formation, undated) above
these lowermost rift fills show an association with localized compressive deformation
along the palaeo-Rand anticline, north of Johannesburg, but again with no evidence
of any major terrane amalgamations with the Kaapvaal. From ca. 2642 to 2432 Ma,
the craton was drowned with a long-lived epeiric marine carbonate-banded iron formation
platform covering much of it and preserved in all three Transvaal Basins (TB).
During this general period, at ca. 2691â2610 Ma, the Kaapvaal Craton collided with a
small exotic terrane [the Central Zone (CZ), Limpopo Belt] in the north. Although farfield
tectonic effects are likely implicit in TB geodynamics, again there is no
case to be made for supercontinent formation. Following an 80â200 million years
(?) hiatus, with localized deformation and removal of large thicknesses of chemically
precipitated sediments along the palaeo-Rand anticline, the uppermost Pretoria Group of the Transvaal Supergroup was deposited. This reflects two episodes of
rifting associated with volcanism, and subsequent thermal subsidence within a sag
basin setting; an association of the second such event with flood basalts supports
a plume affinity. At ca. 2050 Ma the Bushveld Complex intruded the northern
Kaapvaal Craton and reflects a major plume, following which KaapvaalâCZ
collided with the Zimbabwe Craton, when for the first time, strong evidence exists for a
small supercontinent assembly, at ca. 2.0 Ga. We postulate that the long-lived evidence
in favour of active mantle (cf. plume) influences with subordinate and localized tectonic
shortening, implicit within the review of ca. 3.1â2.05 Ga geological history of the
Kaapvaal Craton, might reflect the influence of earlier Precambrian mantle-dominated
thermal systems, at least for this craton.University of Pretoria and the National Research Foundation of South
Africa.http://www.tandfonline.com/loi/tigr20nf201