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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright AUTHOR'S PERSONAL COPY Journal of African Earth Sciences 83 (2013) 74–103 Contents lists available at SciVerse ScienceDirect Journ al of African Earth Scien ces journal homepage: www.elsevier.com/locate/jafrearsci Variations in eruptive style and depositional processes of Neoproterozoic terrestrial volcano-sedimentary successions in the Hamid area, North Eastern Desert, Egypt Ezz El Din Abdel Hakim Khalaf Cairo University, Faculty of Science, Geology Department, Giza, Egypt article info abstract Article history: Two contrasting Neoproterozoic volc ano-sedimentary successions ofca. 600 m thickness were recog- Received 6 June 2012 nized in the Hamid area, Northeastern Desert, Egypt. A lower Hamid succession consists of alluvial sed- Received in revised form 29 November 2012 iments, coherent lava flows, pyroclastic fall and flow deposits. An upper Hamid succession includes Accepted 20 February 2013 deposits from pyrocla stic density currents, sills, and dykes. Sedimentological studies at different scales Available online 4 March 2013 in the Hamid area show a very comp lex interaction of fluvial, eruptive, and gravitational processes in time and space and thus provided meaningful insights into the evolution of the rift sedimen tary environ- Keywords: ments and the identification of different stages of effusive activity, explosive activity, and relative quies- Hamid area cence, determining syn-eruptive and inter-eruptive rock units. Volcano-sedimentary successions Facies analysis The volcano-se dimentary deposits ofthe study area can be ascribed to 14 facies and 7 facies associa- Vent and eruption column dynamics tions: (1) basin-border alluvial fan, (2) mixed sandy fluvial braid plain, (3) bed-load-dominate dephem- Basin-fill architecture eral lake, (4) lava flows and volc aniclastics,(5) pyroclastic fall deposits, (6) phreatomagmatic volcanic deposits, and (7) pyroclastic density current deposits. These systems are in part coeval and in part suc- ceed each other, forming five phases of basin evolution: (i) an opening phase including alluvial fan and valley flooding together with a lacustrine perio d,(ii) a phase of effusive and explosive volcanism (pulsa- tory phase), (iii) a phase of predominant explosive and deposition from base surges (collapsing phase), and (iv) a phase of caldera eruption and ignimbrite-forming processes (climactic phase). The facies archi- tectures record a chan gein volcanic activity from mainly phreatomagmatic eruptions, producing large volumes of lava flows and pyroclastics (pulsatory and collapsing phase), to highly explosive, pumice-rich plinian-type pyroclastic density current deposits (climactic phase). Hamid area is a small-volume vol- cano, however, its magma compositions, eruption styles, and inter-eruptive breaks suggest, that it closely resembles a volc anic archite cture commonly associated with large, composite volcanoe s. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction valuable information on the location or direction of the source vent and the changing eruption styles of a volcano that has since Explosive volcanic eruptions commonly result in thick accu- been removed by erosion or buried in the subsurface (Kataoka mulation of pyroclastic debris over wide areas and instantaneous et al., 2001; Lipman, 1976; Németh, 2010; Riggs and Busby-Spera modification of the topography and drainage networks around a (1990)). They also provide excellent chronostratigraphic marker volcano (Manville et al., 2009; Németh et al., 2012; Smith, horizons that can help correlate isolated sedimentary sections 1991). Large volumes of pyroclastic debris are also delivered and unravel complex sedimentary facies architectures (Karaoglu rapidly to nearby sedimentary basins and lowlands, forming and Helvaci, 2012; Knott et al., 2005). ‘‘syneruption’’ (Smith, 1991) or ‘‘posteruption’’ (Manville, 2001 ) Upward volcano growth, combined with tectonic uplift pro- deposits during and immediately after major eruptions. The duces prograding coarsening-upwards volcaniclastic successions influence of explosive volcanism on sedimentation, especially in to P1 km thick in intra-arc or arc-marginal basins. Accumulations nonmarine settings, has been therefore investigated in a number are thickest close to each volcano and fine and thin distally, giving of studies (Kataoka, 2005; Mcclaughry and Gaylord, 2005; Palmer rise to proximal–distal facies patterns (Kano and Takarada, 2007; and Shawkey, 1997; Smith, 1991; Smith et al., 2002). The volcani- Smith, 1991; Zernack et al., 2009 ). Proximal facies corresponding clastic deposits in a sedimentary basin can in turn provide to the volcanic edifice comprise lava flows, autoclastic, and pyro- clastic breccias and hypabyssal intrusions and pass laterally into E-mail address: [email protected] medial apron associations of pyroclastic-flow, debris-avalanche, 1464-343X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jafrearsci.2013.02.009 AUTHOR'S PERSONAL COPY E.E.D.A.H. Khalaf / Journal of African Earth Sciences 83 (2013) 74–103 75 debris- and hyperconcentrated flow (lahar) deposits and then sandstone of Nubia facies. Publications on the North Eastern Desert distal facies association of braided to meandering fluvial system (NED), including the study area, are few in number if compared deposits, overbank alluvium, and interbedded tephras (Kataoka, with the published works on the central and southern parts of 2005; Maeno and Taniguchi, 2009). In weakly extensional, low-re- the Eastern Desert (e.g., Dardir and Abu Zied, 1972; Ghanem lief intra-arc basins, andesitic–dacitic stratovolcanoes may co-exist et al., 1973; Khalaf, 1999; Mohamed et al., 2000 ). Previous works with intermediate-silicic composition calderas, resulting in com- focused on the larger scale implications involving geochemistry, plex intercalations of volcaniclastic rocks, lavas, and intrusions of geotectonic setting, and very few radiogenic dating of the vol- varying composition and depositional environment (Smith et al., cano-sedimentary successions in Egypt (Breitkreuz et al., 2010; 1993; White and Robinson, 1992). Willis et al., 1988; Wilde and Youssef, 2002). Comparatively little The Hamid area (600 km2) is located between 26°580 and is known about the internal lithofacies subdivisions of the eruptive 27°100N latitude and 32°500 and 33°020E longitude (Fig. 1). Wadi sequence,volcanological , and sedimentological facies analysis and Hamid area, known for its Pb-mining activity, lies to the west of the implications for the eruption style, transport, and depositional Gabal Dokhan and is occupied by the Neoproterozoic Dokhan- processes. New field observations and a stratigraphy for the inter- Hammamat volcano-sedimentary successions (Fig. 1). These suc- nal subdivision of the Neoproterozoic volcano-sedimentary succes- cessions are unconformably overlain from the west by Phanerozoic sions in the Hamid area, based on regional mapping, extensive Fig. 1. Simplified geologic map showing the distribution of the Dokhan Volcanics and Hammamat Group in the North Eastern Desert, Egypt (data were collected from several resources including Abdel Rahman, 1996; Hassan and Hashad, 1990; Grothaus et al., 1979; Wilde and Youssef, 2002 ). The approximate boundary between North Eastern Desert (NED), Central Eastern Desert (CED), and South Eastern Desert (SED) according to Greiling et al. (1994). AUTHOR'S PERSONAL COPY 76 E.E.D.A.H. Khalaf / Journal of African Earth Sciences 83 (2013) 74–103 detailed stratigraphic logging, and petrographic analysis are pre- 2. Geological setting sented. Hence, this paper aims to describe a detailed sedimentolog- ical and volcanological facies analysis for the volcano-sedimentary Based on the field relations and radiogenic ages, Stern and successions and interpret them in terms of depositional systems Hedge (1985) geologically subdivided the Egyptian Eastern Desert and external controls on deposition, proposing a model for the into a threefold division namely: northern (NED), central (CED), stratigraphic evolution of continental basins dominated by subaer- and southern (SED) parts (Fig. 1). They mentioned that the oldest ial deposits. In the following, this study focuses on the most repre- rocks are concentrated in the southern part, while the youngest sentative part of the N–S oriented volcano-sedimentary succession units comprising the Dokhan volcanics, the Hammamat sediments, in the study area (Fig. 2), where the continuity and massive char- and young granites as well as dyke swarms are common in the acter of the exposure and the preservation of many original igne- northern part. The NED–CED boundary is generally represented ous and sedimentary features permit a nearly complete study of by a straight, curved or complexly curvilinear N60 °E trending its anatomy. thrust