SEQUENCES AND FACIES CHARACTERISTICS OF
PYROCLASTIC ROCKS, UMM GURUF AREA, NORTHERN
EASTERN DESERT, EGYPT
EZZ EL DIN ABDEL HAKIM KHALAF
Dept. Geology, Fac.Sciences, Cairo Univ., Giza, Egypt
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Umm Guruf volcanics is the largest andesitic stratovolcanics. Their edifice is steeply dipping, multi-episodic of predominantly lava flows interbedded and overlain by pyroclastic deposits. The style of explosive activity varied from mainly phreatomagmatic eruptions, producing large volume of volcanic flows and pyroclastics to highly explosive plinian and pumice flow deposits-forming eruptions. Pyroclastic deposits are genetically and lithologically divided into ground surge deposits (?), pyroclastic fall and flow deposits. Planar stratification with low angle cross bedding and crossing gas pipe characterize the surge deposits. Fall deposits are generally bedded with well to moderate sorting, in constrast to flow deposits. in which bedding is not observed. Pyroclastic flow deposits are the most voluminous volcanic products in the area. They are composed of more than one flow unit, each represented in the field by a discrete depositional flow unit. Co-ignimbrite breccia lag deposits. pumice flow deposits (welded lithic-poor and non-welded lithic-rich), sillar and ash flow deposits are common different facies observed within pyroclastic flow deposits. Pumice flow deposits are characterized by reverse grade bedding and compositional zoning. The combined textural evidences like quench texture, high vesicularity, fragmental nature and large clast size in some pyroclastic deposits suggest that the pyroclastic deposits were erupted in subaerial environment and formed in magmatic arc at continental margin.
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