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PALEOENVIRONMENT OF CARBONIFEROUS

CONGLOMERATES, BLACK MARMOT VALLEY,

KHARKHIRRA UUL, MONGOLIA

JO NISSENBAUM

Whitman CollegeSponsor: Patrick Spencer

INTRODUCTIONThe Black Marmot Conglomerate, located in

the eastern Kharkhirra Uul in westernMongolia, is 500 m thick, not including anunknown thickness of coal in the middle, andcrops out in stream cuts and on steep slopes ofthe Y Valley (Fig. 1). The steeply-dipping

beds contain conglomerate types of both

shallow marine and fluvial origins. The clastsare a variety of lithologies, including quartzite,quartz, sandstone, greenstone, conglomerate,limestone, and volcanics. There are alsonumerous interbeds of organic-rich shales,coarse-grained sandstones, coals, and tuff-rhyolites (Fig. 2). Regional stratigraphiccontext, conglomerate texture (including grainsize, fabric, and rounding), sedimentarystructures, cement and matrix, stratification,and tectonic setting differentiate depositionalenvironments for each unit, though the entire

unit is interpreted as a high-energyfanglomerate that represents both shallow

marine and fluvial deposition, indicating atransgressive/regressive marine series.

The main purposes of this study are todetermine the paleogeographic context of theformation; to constrain the age of the variousunits; to establish the depositionalenvironment of the various conglomerateunits; and to determine thepaleoenvironmental context of theconglomerate exposures and relate it to thegeologic history of Mongolia and east-centralAsia.

Paleogeography

The late Paleozoic was marked by thecontinental collision of a number of small

Figure 2. An exposure of the BlackMarmot Conglomerate.

Figure 1. Topographic map showing Y-Valleystudy area.


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microcontinents that came together asconvergent tectonics dominated their margins.As the Siberian craton and the North Chinablock converged, ancient continental crust wasthrust onto the continental margin and smallisland arcs, subduction wedges, and ophioliticbelts were accreted as preexisting basementrock was deformed and faulted, and upliftinitiated. This boundary, known as theMongolian-Okhostsk Mobile Zone, isrepresented by 8 distinct and varied orogenicbelts, or superterranes, making Mongoliasimilar to that of a complex accretionary-collision system (Misar, 1997).

The northeastern portion of the KharkhirraUul area is located near the border of two ofthese 8 large orogenic superterranes, the North

Mongolian Early Caledonides and theMongolian Altai Caledonides. The smallersubterranes that the Black Marmot study areastraddles are the Hovd Turbidite Terrane andthe lgii Oceanic Terrane. Combined, thesesubterranes represent a transgressive sequenceof Lower to Upper Paleozoic marine tuff-terrigenous sediments with synorogenic-postorogenic molasse sediments and localcontinental volcanics (Tomurtogoo, 2003;Misar, 1997). Based on geochemical andpetrologic data of these continental volcanics,it is probable that the Black Marmot stratawere deposited in an extensional back-arcbasin formed at the time the island arcs of theMongolian Altai Caledonides were collidingwith the North Mongolian Early Caledonides.

Regional Stratigraphy

The regional stratigraphy of the Black Marmotcoal mine is a complex of middle Paleozoic

marine-terrestrial rocks that overlie the EarlyPaleozoic Lakes Island Arc Terrane. Douglas(cf., this volume) found that this 10-km2region consists of five distinct lithologic units.The oldest units are turbidites and organic-richshale, interbedded with dolostone, mudstone,and limestone. These units are overlain by alithic sandstone and conglomerate series, andcapped by a conglomerate with prominentinterbeds of shale, sandstone, coal, and tuff-rhyolite. In a regional context, the

stratigraphy appears to represent a transitional

system that fluctuated between a deepermarine environment, in which the turbiditeswere deposited, and progressively shallowerenvironments, in which the limestones, shales,and sandstones were deposited, which gradedinto an estuarine-terrestrial setting, in whichthe conglomerates were deposited. Thesystem therefore contains lithologic units thatwere deposited during the rise and fall of sealevel. This oscillation of sea level may be dueto glaciation in Gondwanaland as well asisostatic change resulting from the impact ofthe North China block and the Siberian craton.

Age And Correlation

The Black Marmot Conglomerate has not beenpreviously mapped, and therefore there is no

age determination for this section. It washoped that there would be fossils within thestrata; however, few were found. Fragmentsof a fossil questionably assigned asSphenopsida, or Horsetail, were the onlyfossils present, found within shale and coalinterbeds. Unfortunately, no age constraintswere established since the Sphenopsida rangefrom the middle Devonian to present.However, during the early Carboniferous, agreater diversity of distinctly sphenopsid

plants became more prominent which suggeststhat a Carboniferous age for the conglomeratesis appropriate (Stewart and Rothwell, 1993).

Preliminary Ar40/Ar39 dating of pillow basaltsimmediately before the conglomerate unitsyielded a date of 300 +/- 4 Ma, placing themin the late Carboniferous (Layer and Drake,Geochronology Laboratory, University ofAlaska, Fairbanks, 2005). Because thesebasalts are stratigraphically below the

conglomerates, it can be inferred that theconglomerates were deposited after this time,most likely in the later Carboniferous to earlyPermian. However, excess argon in thesamples makes these dates questionable.

Paleoenvironment

The results presented here are based onmeasured outcrop sections with a cumulativethickness of more than 500-m (Fig. 3). Thereare noteworthy depositional differences within

each of the 35 conglomerate units includingclast size, clast composition, imbrication,


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sedimentary structures, matrix, and cement.Most of the conglomerates have polymodalclasts, ranging from 30 cm to 1 cm and are

angular to sub-rounded; very few have anywell-rounded clasts. The majority of the units

are oligomict-orthoconglomerates with littlematrix and a grain-supported framework withlittle to no internally organized fabric (Fig. 4).

The fact that most of these rocks areoligomicts implies decomposition and

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.


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disintegration of large amounts of rock with aclimate and topography that promoteschemical decomposition of all but the mostresistant components. The units that are notoligomicts are paraconglomerates with amatrix-supported framework. These units,like the oligomicts, lack any internalorganization.

Though several of the conglomerate unitsshow slight imbrication, none of them showany other sedimentary structure. Anotherdifference in the conglomerate beds is thematrix. Approximately 15 of theconglomerates have a carbonate matrix, whilethe rest are composed of silica. Thedifferences in the depositional environmentsof these different matrices is unknown.

Based on these observations, the majority ofthe conglomerates contain clasts that weretransported fairly short distances anddeposited by processes of mass wasting, suchas debris flows, while few of the conglomerateunits were deposited by streams into a subtlychanging nearshore environment.

The interbeds of the Black MarmotConglomerate also represent variable settingswithin a transgressive-regressive sequence.The sandstone interbeds represent shallow,nearshore deposition of compositionallymature and texturally submature sands. Theshale interbeds were deposited in slightlydeeper water. The coal interbeds indicate aheavily vegetated marginal marine setting in awarm, moist climate where rapid burial of theorganics prevented oxidation (Fig. 5). Thisusually occurs in swampy or boggy settings.

The various lenses of siltstone within the coal

and sedimentary structures within these lensescould represent small areas of a marginalmarine environment that received variableamounts of additional water that deposited siltas they entered.

DISCUSSIONThe Black Marmot conglomerates are rocks,suspected to be Carboniferous, that were mostlikely deposited in shallow, equatorial seas.The stratigraphy of the conglomeratesindicates that relative sea level was oscillatingfrequently, creating a fluctuating estuarineenvironment in which deposition likelyoccurred. This oscillation may have beencaused by interactions between glaciation andisostatic change in elevation.

Based on the types of rocks found as well asthe regional tectonics of the area, it is likely

that the deposition of these conglomeratesoccurred in an extensional back-arc basin,though more research must be done to gain amore comprehensive understanding of thenature of the tectonic setting.

The research in the summer of 2004 hasyielded some evidence in order to determinepaleoenvironment of the Black MarmotConglomerate, however, more research issuggested. In particular, it is suggested that

the source area of some of the clasts in theconglomerates is examined. While many ofthe clasts could have come from the localstratigraphy, a number of clasts, such asquartzite, are not recognized locally. Further,granites, a locally occurring rock, are notfound within any of the conglomerates. Moreresearch should also be done to further explainthe structure of the area, which appears highlyfolded.

Figure 5. Black Marmot coal mine.


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REFERENCES CITEDMisar, Z., 1997, Mongolia, in E. M. Moores and R.W.Fairbridge, eds., Encyclopedia of European and Asianregional geology: New York, Chapman and Hall, p.540-544.

Stewart, W. N. and Rothwell, G. W., 1993, Paleobotanyand the evolution of plants: Cambridge UniversityPress, Cambridge, England.

Tomurtogoo, O., ed., 2003, Tectonic map of Mongolia:Mongolian Academy of Sciences, scale 1:1,000,000.

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