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Paleoenvironment of Carboniferous Conglomerate, Kharkhiraa

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Paleoenvironment of Carboniferous Conglomerate, Kharkhiraa http://keck.wooster.edu/publications/eighteenthannual… PALEOENVIRONMENT OF CARBONIFEROUS CONGLOMERATES, BLACK MARMOT VALLEY, KHARKHIRRA UUL, MONGOLIA JO NISSENBAUM Whitman College Sponsor: Patrick Spencer INTRODUCTION unit is interpreted as a high-energy fanglomerate that represents both shallow The Black Marmot Conglomerate, located in marine and fluvial deposition, indicating a the eastern Kharkhirra Uul in western transgressive/regressive marine series. Mongolia, is ≈500 m thick, not including an unknown thickness of coal in the middle, and The main purposes of this study are to crops out in stream cuts and on steep slopes of determine the paleogeographic context of the the Y Valley (Fig. 1). The steeply-dipping formation; to constrain the age of the various units; to establish the depositional environment of the various conglomerate units; and to determine the paleoenvironmental context of the conglomerate exposures and relate it to the geologic history of Mongolia and east-central Asia. Figure 1. Topographic map showing Y-Valley study area. beds contain conglomerate types of both shallow marine and fluvial origins. The clasts are a variety of lithologies, including quartzite, quartz, sandstone, greenstone, conglomerate, limestone, and volcanics. There are also numerous interbeds of organic-rich shales, coarse-grained sandstones, coals, and tuff- Figure 2. An exposure of the Black rhyolites (Fig. 2). Regional stratigraphic Marmot Conglomerate. context, conglomerate texture (including grain size, fabric, and rounding), sedimentary structures, cement and matrix, stratification, Paleogeography and tectonic setting differentiate depositional The late Paleozoic was marked by the environments for each unit, though the entire continental collision of a number of small 117 http://keck.wooster.edu/publications/eighteenthannual… microcontinents that came together as system that fluctuated between a deeper convergent tectonics dominated their margins. marine environment, in which the turbidites As the Siberian craton and the North China were deposited, and progressively shallower block converged, ancient continental crust was environments, in which the limestones, shales, thrust onto the continental margin and small and sandstones were deposited, which graded island arcs, subduction wedges, and ophiolitic into an estuarine-terrestrial setting, in which belts were accreted as preexisting basement the conglomerates were deposited. The rock was deformed and faulted, and uplift system therefore contains lithologic units that initiated. This boundary, known as the were deposited during the rise and fall of sea Mongolian-Okhostsk Mobile Zone, is level. This oscillation of sea level may be due represented by 8 distinct and varied orogenic to glaciation in Gondwanaland as well as belts, or superterranes, making Mongolia isostatic change resulting from the impact of similar to that of a complex accretionary- the North China block and the Siberian craton. collision system (Misar, 1997). Age And Correlation The northeastern portion of the Kharkhirra The Black Marmot Conglomerate has not been Uul area is located near the border of two of previously mapped, and therefore there is no these 8 large orogenic superterranes, the North age determination for this section. It was Mongolian Early Caledonides and the hoped that there would be fossils within the Mongolian Altai Caledonides. The smaller strata; however, few were found. Fragments subterranes that the Black Marmot study area of a fossil questionably assigned as straddles are the Hovd Turbidite Terrane and Sphenopsida, or Horsetail, were the only the Ölgii Oceanic Terrane. Combined, these fossils present, found within shale and coal subterranes represent a transgressive sequence interbeds. Unfortunately, no age constraints of Lower to Upper Paleozoic marine tuff- were established since the Sphenopsida range terrigenous sediments with synorogenic- from the middle Devonian to present. postorogenic molasse sediments and local However, during the early Carboniferous, a continental volcanics (Tomurtogoo, 2003; greater diversity of distinctly sphenopsid Misar, 1997). Based on geochemical and plants became more prominent which suggests petrologic data of these continental volcanics, that a Carboniferous age for the conglomerates it is probable that the Black Marmot strata is appropriate (Stewart and Rothwell, 1993). were deposited in an extensional back-arc 40 39 basin formed at the time the island arcs of the Preliminary Ar /Ar dating of pillow basalts Mongolian Altai Caledonides were colliding immediately before the conglomerate units with the North Mongolian Early Caledonides. yielded a date of 300 +/- 4 Ma, placing them in the late Carboniferous (Layer and Drake, Regional Stratigraphy Geochronology Laboratory, University of The regional stratigraphy of the Black Marmot Alaska, Fairbanks, 2005). Because these coal mine is a complex of middle Paleozoic basalts are stratigraphically below the marine-terrestrial rocks that overlie the Early conglomerates, it can be inferred that the Paleozoic Lakes Island Arc Terrane. Douglas conglomerates were deposited after this time, (cf., this volume) found that this 10-km2 most likely in the later Carboniferous to early region consists of five distinct lithologic units. Permian. However, excess argon in the The oldest units are turbidites and organic-rich samples makes these dates questionable. shale, interbedded with dolostone, mudstone, and limestone. These units are overlain by a Paleoenvironment lithic sandstone and conglomerate series, and The results presented here are based on capped by a conglomerate with prominent measured outcrop sections with a cumulative interbeds of shale, sandstone, coal, and tuff- thickness of more than 500-m (Fig. 3). There rhyolite. In a regional context, the are noteworthy depositional differences within stratigraphy appears to represent a transitional each of the 35 conglomerate units including clast size, clast composition, imbrication, 118 http://keck.wooster.edu/publications/eighteenthannual… Figure 3. Detailed stratigraphic column of the Black Marmot Conglomerate and interbeds, including the large coal mine with interbeds of siltstone and limonite. Pictures represent the sane units in the stratigraphic column. s edimentary structures, matrix, and cement. are oligomict-orthoconglomerates with little Most of the conglomerates have polymodal matrix and a grain-supported framework with clasts, ranging from 30 cm to 1 cm and are little to no internally organized fabric (Fig. 4). angular to sub-rounded; very few have any The fact that most of these rocks are well-rounded clasts. The majority of the units oligomicts implies decomposition and 119 http://keck.wooster.edu/publications/eighteenthannual… disintegration of large amounts of rock with a climate and topography that promotes chemical decomposition of all but the most resistant components. The units that are not oligomicts are paraconglomerates with a matrix-supported framework. These units, like the oligomicts, lack any internal Figure 5. Black Marmot coal mine. organization. and sedimentary structures within these lenses could represent small areas of a marginal marine environment that received variable amounts of additional water that deposited silt as they entered. DISCUSSION The Black Marmot conglomerates are rocks, suspected to be Carboniferous, that were most likely deposited in shallow, equatorial seas. The stratigraphy of the conglomerates indicates that relative sea level was oscillating Though several of the conglomerate units frequently, creating a fluctuating estuarine show slight imbrication, none of them show environment in which deposition likely any other sedimentary structure. Another occurred. This oscillation may have been difference in the conglomerate beds is the caused by interactions between glaciation and matrix. Approximately 15 of the isostatic change in elevation. conglomerates have a carbonate matrix, while the rest are composed of silica. The Based on the types of rocks found as well as differences in the depositional environments the regional tectonics of the area, it is likely of these different matrices is unknown. that the deposition of these conglomerates occurred in an extensional back-arc basin, Based on these observations, the majority of though more research must be done to gain a the conglomerates contain clasts that were more comprehensive understanding of the transported fairly short distances and nature of the tectonic setting. deposited by processes of mass wasting, such as debris flows, while few of the conglomerate The research in the summer of 2004 has units were deposited by streams into a subtly yielded some evidence in order to determine changing nearshore environment. paleoenvironment of the Black Marmot Conglomerate, however, more research is The interbeds of the Black Marmot suggested. In particular, it is suggested that Conglomerate also represent variable settings the source area of some of the clasts in the within a transgressive-regressive sequence. conglomerates is examined. While many of The sandstone interbeds represent shallow, the clasts could have come from the local nearshore deposition of compositionally stratigraphy, a number of clasts, such as mature and texturally submature sands. The quartzite, are not recognized locally. Further, shale interbeds were deposited in slightly granites, a locally occurring rock, are not deeper water. The coal interbeds indicate a found within any of the conglomerates.
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