Volcaniclastic aggradation in El Tarr Area, Southeastern Sinai, Egypt : petrological and geochemical evidence

الملخص EN

The Tarr complex forms the southernmost portion of the Kid Group in the Southeast Sinai Peninsula.

This complex consists of diverse interstratified volcanic and volcaniclastic sedimentary strata and cogenetic hypabyssal intrusive rocks of Late Proterozoic age.

The occurrence of slightly preserved glass shards and volcanic clasts within volcaniclastic sediments indicate a narrow time span between eruption and final deposition by resedimentation.

The volcano-sedimentary sequence of El Tarr area displays a low metamorphic grade and is nearly undeformed.

These volcaniclastic rocks are characterized by rapid lateral and vertical facies and petrofacies variation.

Modal analysis of these rocks defines a source area comprising mainly volcanic and intrusive rocks, with no indication of continental detritus or detritus from exotic sources. The studied sequence is divided into three stratigraphic intervals on the basis of the predominant lithosomes in each of the intervals.

The lowermost stratigraphic interval contains metamorphosed felsic pyroclastic rocks with marble intercalations, continues vertically to mafic and intermediate volcanic flows, pyroclastic fall and flow deposits and local interbeds of clast-supported conglomerates, associated with cross-laminated sandstones with lensoidal layers of tuffaceous mudstones. These rocks were deposited in a low relief, proximal alluvial fan volcanic apron environment.

The middle stratigraphic interval consists of thick massive to bedded coarse to medium-grained sandstones and interbeds of tuffaceous mud/siltstones, with lensoidal beds of metamorphosed mafic volcanics.

Rocks in the middle interval were deposited in distal alluvial fan volcanic apron environment.

The upper stratigraphic interval contains matrix/clast-supported conglomerates of mixed sources that were deposited in poorly confined, shallow proximal braided river channels on a fluvial plain.

Tectonics are perceived to be a major control on the positioning and long-term evolution of alluvial systems.

By increasing slope gradients through uplift and tilting, or by changing local base-level, incision can be stimulated leading to a switch in the active area of sedimentation.

The switch from alluvial fan to braided river sedimentation and the spatially variable patterns of incision into the alluvial fan bodies can be accounted for by a phase of deformation which affected the El Tarr volcaniclastic rocks during Late Proterozoic time.

A lack of pedogenic modification throughout most of the channelized sediments suggests high rates of sedimentation which inhibited soil development.

The whole studied sequence is similar to rock units in arc-flank volcanic complexes which are interpreted to have accumulated principally on river flood plain and alluvial fan and local aeolian environment.