Abstract

The latest Middle Proterozoic Hazel Formation near Van Horn in the Trans-Pecos region of west Texas is an orogenic elastic wedge that provides insight into paleotectonic evolution of the southern margin of the Laurentian craton. The Hazel Formation is >2.5 km thick adjacent to the contemporaneous Streeruwitz thrust fault and consists exclusively of terrestrial sedimentary rocks. Boulder-sized clasts in conglomerate were deposited within alluvial fans north of the Streeruwitz thrust sheet, 0along the southern margin of a basin of unknown geometry and extent. The alluvial-fan conglomerate interfingers with eolian sedimentary rocks in the northern part of the basin. Alluvial-fan conglomerate megasequences indicate that at least two episodes of faulting accompanied Hazel sedimentation. Alluvial megasequences are overall aggradational within the Hazel succession as revealed by clast size variations within the conglomerate.

Systematic variation of pebble composition up through the Hazel conglomerate provides an erosional history within the source terrain. Carbonate and subordinate mafic volcanic rocks, derived from the pre orogenic Allamoore and Tumbledown Formations, were the principal source off the Hazel Formation. Metavolcanic and metasedimentary rocks from the Carrizo Mountain Group in the present tectonic hinterland are notably absent in the Hazel conglomerate. Moreover, a rhyolite terrain, presently not exposed in the Van Horn area, was a significant component of the Hazel source.

Juxtaposition between source terrain and sedimentary basin, coupled with aggradation of alluvial-fan megasequences along the southern fault-bounded basin margin, supports a transpressive regime for this orogenic belt rather than a purely convergent margin.

Deformation associated with the Hazel orogenic sequence post-dated a 1,126- to 1,070-m.y.-old regional felsic igneous event; Grenville deformation in the Llano uplift of central Texas predated the same igneous event. This implies that the Hazel tectonic episode was a second, later Grenvillian deformation. The terms Llano orogeny and Hazel orogeny are introduced here for the Grenville province in Texas and may be comparable with the Elzevirian and Ottawan orogenies within the Grenville province of Canada.

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