Sedimentology and provenance of Mesozoic nonmarine strata in western Mongolia: A record of intracontinental deformation

Abstract

Western Mongolia is a structurally complicated and little-studied portion of the central Asia tectonic collage, yet contains well-exposed sequences ofMesozoic sedimentary strata that preserve an important record of ancient intraplate deformation. In order to document the record of Mesozoic sedimentary basin development and to provide a basis for interpreting the sedimentary record of intraplate deformation, we studied the sedimentology and provenance of Mesozoic strata at four locations in western Mongolia. Triassic sedimentary strata are missing across the study area, and Lower Jurassic strata unconformably overlie older rocks of various ages. Lower to Middle Jurassic basin fill in the northern portion of the study area is dominated by coarse, granite-bearing conglomerate and arkosic sandstone (Qm 35 F 49 L 16 ) with a volcanic-dominated lithic fraction (Qp 22 Lvm 63 Lsm 15 ). Paleocurrent indicators suggest southwest-directed sediment transport, reflecting uplift of an ancestral version of the Hanhöhiy Uul, a mountain range located north and east of the study area and composed mainly of Cambrian granite and associated volcanics. In the central portion of the study area, Lower to Middle Jurassic strata comprise an upward-fining kilometer-thick sequence from coarse matrix-supported fanglomerate to fine sandstone, shale, and coal suggestive of a mudload-dominated, meandering fluvial environment. Conglomerate clasts are dominantly basalt and granite, and sandstone is arkosic with higher QmFL%L and Qp-Lvm-Lsm%Lsm than equivalent sandstone to the north (Qm 41 F 35 L 24 ; Qp 24 Lvm 41 Lsm 35 ). Paleocurrent indicators suggest transport to the south-southwest. In the southernmost part of the study area, Lower through Upper Jurassic strata record a transition from mudload-dominated fluvial environments with abundant organic matter to fluvial flood-plain and/or mud flat with little organic matter and abundant calcisols. These strata are sharply overlain by

Mongolian coal-bearing basins: Geological settings, coal characteristics, distribution, and resources

“…This might imply a decreased magnitude of crustal extension to the west. Our view on the Mesozoic period tectonic regime in the Valley of Great Lakes is consistent with that of Sjostrom et al (2001). However, Howard et al (2003) proposes extensional conditions.…”

“…In the Lower-Middle Jurassic non-marine, sedimentary coal-bearing sequences more than 2500 m thick accumulated. Based on sedimentological and Sm-Nd isotope analysis, Sjostrom et al (2001) conclude that Jurassic strata in the Valley of Great Lakes in the Western Mongolia province were deposited in flexural basin. Another possibility for the origin of the basin is a back-arc extension behind an arc that faced north toward the Mongol-Okhotsk ocean.…”

“…(Markova, 1975;Durante, 1976). A summarized list of previously studied flora, insects, and mollusks from the Jargalant formation can be found in Sjostrom et al (2001).…”

“…During the last decade, several key papers on Mongolian coal geology have been published (e.g. Bat-Erdene, 2001;Bat-Erdene et al, 2001;Jargal et al, 2002) and several Mongolian/Western scientists have focused on the basement geology in terms of the collage of tectonostratigraphic terranes, the history of the intracontinental deformation, and the sedimentary basins of Mongolia (e.g., Hendrix et al, 1996;Lamb and Badarch, 1997;Graham et al, 2001;Johnson et al, 2001;Sjostrom et al, 2001;Badarch et al, 2002;Johnson, 2004). In this paper, we present an up-to-date synthesis of the Mongolian coal-bearing provinces and basins including data on tectonic settings, depositional environments, stratigraphy, coal quality, petrographical compositions and reserves.…”

A short-lived but significant Mongol–Okhotsk collisional orogeny in latest Jurassic–earliest Cretaceous

“…At the Dariv locality, western Mongolia, the Upper Jurassic Dariv Formation is composed of sandy siltstone and calcisol-bearing mudstone (Sjostrom et al, 2001;Howard et al, 2006) (Figs. 1, 2, 3).…”