Closure of the Proto-Tethys Ocean and Early Paleozoic amalgamation of microcontinental blocks in East Asia

From static to dynamic provenance analysis—Sedimentary petrology upgraded

Petrographic classification of sand and sandstone

The modern Nile sediment system: Processes and products

We often resort to mythical thinking in the face of natural phenomena that we hardly understand. Sedimentary petrology has several long-standing myths. One is that sediments increase their “maturity” through time by winnowing and sorting, physical wear, chemical weathering, diagenesis, and recycling, as if their destiny were to reach a final stage of perfection represented by quartz spheres of equal size. Such an armchair representation of supermature purity is embodied in white recycled desert sand, whereas a modern analogue for its opposite, the dark dirty rock improperly called graywacke, is yet to be found. Clay matrix does not occur in sand deposited from tractive currents, which is sorted at the instant of settling. Sand grains get rounded relatively rapidly during transport in air but very slowly when transported in water. The degree of winnowing, sorting, and rounding (“textural maturity”) has little to do with the accumulation of “modifying energy” through time. “Mineralogical maturity” does not increase notably even after thousands of kilometers of transport in high-energy environments, and not even during recycling, a physical process that in a weathering-limited denudation regime tends at most to reproduce the mineralogy of parent clastic rocks. Rather, “mineralogical maturity” decreases drastically where sedimentary rock fragments are eroded in abundance from mudrocks interbedded in the source, and sand recycled entirely from orogenic flysch or molasse may contain less than 10% quartz. If natural sediment factories were quartz distilleries, then quartzose sand should become more common through geological time. Pure quartz sand, however, is relatively rare in the modern world, and largely confined to hyperhumid equatorial latitudes or recycled eolian dunefields. A relative increase in more durable minerals is achieved effectively only by chemical processes such as selective leaching in soils, or especially during burial diagenesis, when the time and temperature available for chemical reactions are much longer and higher than at the Earth9s surface. If recycled sediments are commonly quartz-rich with very poor assemblages dominated by zircon, tourmaline, and rutile, it is because their parent sandstones have undergone diagenetic dissolution prolonged for many millions of years at temperatures of many tens of degrees before final exhumation and erosion.

The maturity myth in Sedimentology and provenance analysis

http://meetingorganizer.copernicus.org/EGU2013/EGU2013-1285.pdf

Provenance and recycling of Arabian desert sand

This study seeks to determine the ultimate origin of aeolian sand in Arabian deserts by high-resolution petrographic and heavy-mineral techniques combined with zircon U–Pb geochronology. Point-counting is used here as the sole method by which unbiased volume percentages of heavy minerals can be obtained. A comprehensive analysis of river and wadi sands from the Red Sea to the Bitlis-Zagros orogen allowed us to characterize all potential sediment sources, and thus to quantitatively constrain provenance of Arabian dune fields. Two main types of aeolian sand can be distinguished. Quartzose sands with very poor heavy-mineral suites including zircon occupy most of the region comprising the Great Nafud and Rub' al-Khali Sand Seas, and are largely recycled from thick Lower Palaeozoic quartzarenites with very minor first-cycle contributions from Precambrian basement, Mesozoic carbonate rocks, or Neogene basalts. Instead, carbonaticlastic sands with richer lithic and heavy-mineral populations characterize coastal dunes bordering the Arabian Gulf from the Jafurah Sand Sea of Saudi Arabia to the United Arab Emirates. The similarity with detritus carried by the axial Tigris–Euphrates system and by transverse rivers draining carbonate rocks of the Zagros indicates that Arabian coastal dunes largely consist of far-travelled sand, deposited on the exposed floor of the Gulf during Pleistocene lowstands and blown inland by dominant Shamal northerly winds. A dataset of detrital zircon U–Pb ages measured on twelve dune samples and two Lower Palaeozoic sandstones yielded fourteen identical age spectra. The age distributions all show a major Neoproterozoic peak corresponding to the Pan-African magmatic and tectonic events by which the Arabian Shield was assembled, with minor late Palaeoproterozoic and Neoarchean peaks. A similar U–Pb signature characterizes also Jafurah dune sands, suggesting that zircons are dominantly derived from interior Arabia, possibly deflated from the Wadi al-Batin fossil alluvial fan or even from Mesozoic sandstones of the Arabian margin accreted to the Cenozoic Zagros orogen. Due to extensive recycling and the fact that zircon is so resistant to weathering and erosion, the U–Pb age signatures are much less powerful a tracer of sedimentary provenance than framework petrography and heavy minerals. Actualistic provenance studies of dune fields at subcontinental scale shed light on the generation and homogenization of aeolian sand, and allow us to trace complex pathways of multistep sediment transport, thus providing crucial independent information for accurate palaeogeographic and palaeoclimatic reconstructions.

Kate Allen

Papers

Provenance and recycling of Arabian desert sand

Provenance and recycling of Arabian desert sand