DUST IN THE WIND: AEOLIAN SEDIMENT IN MIDDLE ORDOVICIAN CARBONATES OF NORTH AMERICA*Ronald R. McDowell , West Virginia Geological and Economic Survey, Morgantown, WV, 26508( )mcdowell@geosrv.wvnet.edu

NEALMONT LIMESTONE

View of Frank’s Run just north of Blue Grass, VA where the NealmontLimestone is reasonably well exposed. Note: a thrust fault boundsthe east side of the valley and places the Nealmont on top of theOrdovician Reedsville Formation.

Outcrop of Nealmont Limestone north of Bluegrass, VA. Thin-beddedmicrites with light tan, silty shale interbeds are typical in this area.

Bioturbated bedding surface in the Nealmont showingorange silt and clay fillings in burrows. Coin is 3 cmin diameter.

Bioturbated micrite, typically containing the trace fossil , isan identifying feature of the Nealmont in the study area. Burrows arepartially or completely filled with light tan to orange silt and clay whichhas oxidized to give the coloration. Coin is 3 cm in diameter.

Chondrites

Thin section photomicrograph taken from the sample above showingiron oxide coloration and quartz silt in the burrow fill. Note the quartzsilt (arrows) scattered throughout the micritic matrix of the rock. Plainpolarized light - crossed polars.

1 mm

Approximate extent of Ordovician carbonates (shading) including theNealmont Limestone encountered by the author during various STATEMAPprojects undertaken in eastern West Virginia and western Virginia. The typelocality (star) for the Nealmont is at Union Furnace, near Huntingdon, PA.

Although the USGS Stratigraphic Lexicon still lists the Nealmont Lime-stone as “Middle Ordovician Rocklandian” in age, that assessmentappears to be incorrect. Controversy from the early 1990’s (see Ryderand others, 1992 and Haynes, 1992) stems from disagreement betweenthe North American and British biostratigraphic zonations for the Ordo-vician. Ultimately, the Rocklandian Stage was moved into the LateOrdovician and the age of the Middle Ordovician has been revised.

*

Stratigraphic nomenclature and age relationships for the Kanosh and Nealmontstudy areas taken from AAPG’s COSUNA (Correlation of Stratigraphic Units ofNorth America) charts for the Great Basin (Hintze, 1985) and the Northern Ap-palachian Region (Patchen and others, 1984). The Nealmont here is correctlyassigned to the Late Ordovician. However, there has been a major revision ofstratigraphic zonation for the Ordovician (see below) - the Llandeilo Stage hasbeen eliminated and the age of the Early/Middle Ordovician boundary has beenchanged from 485 Ma to 472 Ma. The deposition of the Kanosh falls near themiddle of graptolite zone which corresponds to thebottom of the graptolite zone (Yuandong andothers, 2009) or approximately 462 Ma. The presence of both the Diecke andMillbrig bentonites within the Nealmont (Haynes, 1992) constrains its depositionto approximately 454 Ma.

Didymograptus murchisoniArchiclimactograptus riddellensis

Western Utah and Eastern Nevada Western Virginia and Eastern West Virginia

Details of Ordovician sea-level changes and the currently accepted stratigraphic zonation and time constraints forthat period. Modified from Haq and Schutter, 2008.

STRATIGRAPHY

ABSTRACT

The Middle Ordovician Nealmont Limestone of eastern West Virginia and western Virginia ismarked bythe presence of numerous specimens of the feeding trace fossil . Theseichnofossils are recognizable from a distance because they are typically infilled with tan or lightorange, quartz silt and clay and stand out in marked contrast to the dark micritic matrix. Inaddition, bedding surfaces may be partially or completely covered with this bioturbated materialin millimeter-thick layers. As a result, these ichnofossils are useful in identifying the Nealmontfor field mapping purposes.

The author has previously had the opportunity to study carbonates of the Middle OrdovicianPogonip Group in the Great Basin, USA. These extensive deposits (particularly the KanoshFormation) from the western margin of the North American continent are similarly marked bythe presence of orange, silty laminae and silt-filled and other feeding traces. Theauthor interpreted these fine-grained siliciclastic sediments as aeolian in origin because of theirpresence along the seaward margin of a basin more than 100 miles from the nearest siliciclasticsediment source. It seems likely that similar sediment in the Nealmont Limestone has acomparable origin.

Reconstruction of continents during the Middle Ordovician places eastern North America atapproximately 10° south latitude and western North America at approximately 15° north latitude,both in a zone of easterly, equatorial wind flow. The source location for deflated sediment isunknown for this time period as the major land masses lay to the south in a nearly polarposition. The presence of deposits of aeolian sediment on opposite sides of the MiddleOrdovician North American continent suggests that it may represent a major sedimentologicalevent with widespread stratigraphic significance.

Chondrites

Chondrites

Type section of the Kanosh Formation at Fossil Mountain in the Con-fusion Range, Millard Co., western Utah.

Kanosh

The Kanosh is a mixed carbonate-clastic unit, generally more sili-ceous in Utah than Nevada. Shown above is an outcrop of biotur-bated Kanosh carbonate in the Ruby Range in eastern Nevada.Orange, siliceous burrow fillings are visible due to oxidized iron inthe fill. Staff is 1.5 m tall.

Intensely bioturbated Kanosh micrite with orange siliceousburrow fillings on a weathered bedding surface in easternNevada. Staff is 1.5 m tall.

Bioturbated Kanosh micrite with orange siliceous burrow fillings ona weathered bedding surface in eastern Nevada. Hammer for scale.

Bioturbated Kanosh micrite with orange siliceous burrow fillings fromeastern Nevada. Hand sample has been cut and polished. Coin is18 mm in diameter.

Kanosh bioherm in central Nevada composed of (alarge calcareous algae), (a small calcareous algae), and on-coids (large, algally coated grains). Siliceous material is not ap-parent. Hammer for scale.

ReceptaculitesNuia

Thin section photomicrograph of Kanosh packstone showing scatteredgrains of quartz silt throughout the rock. The large oncoid in the centerof the slide shows a heavy concentration of quartz silt in the algal layerssurrounding the core of the grain. Cemented with a mixture of sparryand ferroan (pale blue stain) sparry calcite. Plain polarized light.

1 mm

KANOSH FORMATION

Thin section photomicrograph of Kanosh micrite with burrows filled withfecal pellets and pale orange siliceous debris - all cemented with sparrycalcite. Note the quartz silt grains (arrows) isolated within the micriticmatrix. Plain polarized light.

1 mm

Approximate outcrop extent for the Kanosh Formation(shading). The type locality (star) for the Kanosh is atFossil Mountain in the Confusion Range in westernUtah. Figure taken from McDowell, 1987.

Paleogeographic configuration during the period spanning the Middle/LateOrdovician boundary, approximately 459 Ma, appropriate during the depositionof both the Kanosh Formation (red star) and the Nealmont Limestone (greenstar). Modified from MacNiocaill and others, 1997.

Global paleogeographic reconstruction during the period spanning the Middle/Late Ordovician boundary,approximately 459 Ma, showing positions of all land masses during the deposition of both the KanoshFormation (red star) and the Nealmont Limestone (green star). Modified from Blakey, 2003.

Laurentia

Siberia

Baltica Gon

dwan

a

Gondw

ana

Prevailing wind directions at various latitudes. Near-equatorial positionof the depositional areas for both the Kanosh and Nealmont put themwithin the zone of easterly Trade Winds on either side of the equator.Image modified from

.http://library.thinkquest.org/26634/desert/images/

circulate.gif

1000 Km

Where did the fine-grained siliceous sediment found in both the Kanosh and Nealmontcarbonates come from? Both formations appear to be shallow-shelf deposits, and,based on the paleogeographic reconstruction of Blakey (2003), both depositional areasare relatively distant from the nearest exposed lands of the Laurentian continent(~750 km for the Kanosh; ~500 km for the Nealmont). While it is not inconceivable thatfine-grained siliceous material was, at times, moved out onto the shelf by fluvial processes- intervals of silty shale in both the Kanosh and Nealmont do support this - the presenceof more sparsely-distributed quartz silt and iron-rich clays intimately mixed withcarbonates of both formations suggests a different mode of transport. The situation ofboth depositional areas within the zone of equatorial Trade Winds blowing from the eastacross the exposed Laurentian landmass, suggests to the author that windblown fines,deflated from that landmass, may have provided siliceous sedimentary input. This wouldexplain the nearly ubiquitous presence of fine-grained siliceous detritus in bioturbatedmicritic carbonates of both formations and also the inclusion of that detritus (withoutindication of a volume large enough to “smother” or interfere with normal carbonatedeposition) in carbonate bioherms. This siliceous material probably needs an alternativesource of sedimentary influx - aeolian or windblown dust meets those requirements.

Although the Kanosh and Nealmont by revised Ordovician age constraints are no longerboth considered to be Middle Ordovician, their deposition is separated in time by approxi-mately 8 million years. It is not inconceivable that an exposed, unvegetated Laurentianlandmass provided siliceous sediment for aeolian transport over that interval of time.Unfortunately, 8 million years (or longer) does not provide an event well enough con-strained in time to provide a stratigraphic marker.

DISCUSSION AND CONCLUSIONS

MIDDLE-LATE ORDOVICIAN PALEOGEOGRAPHY

REFERENCES

Blakey, R., 2003. Detailed Paleogeography Maps: online resource available at the University of Arizona Department ofGeology, URL: http://jan.ucc.nau.edu/~rcb7/globaltext2.html

Haq, B. and Schutter, S., 2008, A chronology of Paleozoic sea-level changes: Science, v. 322, no. 5898, p. 64-69.

Haynes, J, 1992, Reinterpretation of the Rocklandian (Upper Ordovician) K-Bentonite Stratigraphy in Southwest Virginia,Southeast West Virginia, and Northeast Tennessee: Virginia Division of Mineral Resources Publication 126, 58 p.

Hintze, L. (coordinator), 1985, Correlation of Stratigraphic Units of North America - Great Basin Correlation Chart:American Association of Petroleum Geologists, 1 sheet.

McDowell, R., 1987, Paleogeography, Depositional Environments, and Petroleum Potential of the Middle OrdovicianKanosh Shale, unpublished PhD Thesis, Colorado School of Mines, Golden, 410 p.

Mac Niocaill, C., van der Pluijm, B., and Van der Voo, R., 1997, Ordovician paleogeography and the evolution of theIapetus Ocean: Geology, v. 25, no. 2, p. 159-162.

Patchen, D., Avary, K., and Erwin. R., 1984, Correlation of Stratigraphic Units of North America - Northern AppalachianRegion Correlation Chart: American Association of Petroleum Geologists, 1 sheet.

Stewart, J., 1980, Geology of Nevada: Nevada Bureau of Mines Geological Special Publication, No. 4, 136 p.

Ryder, R., Harris, A., and Repetski, J., 1992, Stratigraphic framework of Cambrian and Ordovician rocks in the centralAppalachian basin from Medina County, Ohio, through southwestern and south-central Pennsylvania to HampshireCounty, West Virginia, Evolution of sedimentary basins; Appalachian basin: U.S. Geological Survey Bulletin, 1839-K, p.K1-K32.

Yuandong, Z., Junxuan, F., Erdtmann, B-D., and Xiao, L., 2009, Darriwilian graptolites of the Shihtien Formation(Ordovician) in west Yunnan, China: Alcheringa, v. 33, p. 303-329.

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ACKNOWLEDGEMENTS

The author acknowledges the U. S. G. S. STATEMAP Program (contract # 01HQAG0040)for financial support while mapping the Ordovician carbonates near Blue Grass, VA.Funding for the author's Doctoral Thesis project in Nevada and Utah was provided in partby NSF Grant EAR-8117322 to R. J. Ross† and in part by Tenneco, Mobil, TexacoResearch, Sohio, and the Department of Geology, Colorado School of Mines.

†Deceased

The author also would like to acknowledge an always cordial association with Fred

Read and his students from Virginia Polytechnic Institute and State University in all

their dealings with the West Virginia Geological and Economic Survey. Fred -

Best Wishes.