Geologic map of the North Coulee Quadrangle, Prairie County, Montana

Abstract Harrat Rahat, one of several large, basalt-dominated volcanic fields in western Saudi Arabia, is a prime example of continental, intraplate volcanism. Excellent exposure makes this an outstanding site to investigate changing volcanic flux and composition through time. We present 93 40Ar/39Ar ages and six 36Cl surface-exposure ages for volcanic deposits throughout northern Harrat Rahat that, when integrated with a new geologic map, define 12 eruptive stages. Exposed volcanic deposits in the study area erupted <1.2 Ma, and 214 of 234 eruptions occurred <570 ka. Two eruptions occurred in the Holocene, including a historically described basalt eruption in 1256 C.E. and a trachyte eruption newly recognized as Holocene (4.2 ± 5.2 ka). An estimated ∼82 km3 (dense rock equivalent) of volcanic product have erupted since 1.2 Ma, though this is a lower limit due to concealment of deposits >570 ka. Over the past 570 k.y., the average eruption rate was 0.14 km3/k.y., but volcanism was episodic with periods alternating between low (0.04–0.06 km3/k.y.) and high (0.1–0.3 km3/k.y.) effusion rates. Before 180 ka, eruptions vented from the volcanic field’s dominant eastern vent axis and from a subsidiary, diffuse, western vent axis. After 180 ka, volcanism focused along the eastern vent axis, and the composition of volcanism varied systematically along its length from basalt dominated in the north to trachyte dominated in the south. We hypothesize that these compositional variations <180 ka reflect the growth of a mafic intrusive complex beneath the southern portion of the vent axis, which led to the development of evolved magmas.

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Preliminary Map Showing Quaternary Geology of the Trafalgar 7.5-Minute Quadrangle, Indiana

Indiana Journal of Earth Sciences ◽

10.14434/ijes.v3i1.31737 ◽

2021 ◽

Vol 3 ◽

Author(s):

Henry Munro Loope

Keyword(s):

Quaternary Geology ◽

Laurentide Ice Sheet ◽

Southern Boundary ◽

Bedrock Topography ◽

The North ◽

Johnson County ◽

Geologic Map ◽

Well Data ◽

Sand And Gravel ◽

Maximum Limit

The Trafalgar 7.5-minute quadrangle is situated near the maximum limit of Wisconsin Episode glacial deposits insouthwestern Johnson County. Till of Wisconsin Episode age (Trafalgar Formation) dominates the surficial geology of thequadrangle. Minor areas of Wisconsin Episode outwash and colluvium are found in valleys and former meltwater pathways.Kame deposits (sand and gravel) are found in the northeast part of the quadrangle and are part of a larger kame complexpresent in central Johnson County. Holocene (post-glacial) alluvium is found in the valleys of the North and South Prongsof Stotts Creek and tributaries. It is also found in two tributary valleys (Barnes Creek and Goose Creek) of Indian Creek inthe southwestern sector of the quadrangle. Unconsolidated sediment thickness generally increases from west to east acrossthe quadrangle, from less than 15 ft along the western margin of the quadrangle to over 150 ft thick in the east-central part ofthe quadrangle. The west to east transition of bedrock topography represents the buried northern end of the KnobstoneEscarpment. Based on water well data, the bedrock topography of the western half of the quadrangle is highly variable (reliefof ca. 75 ft), similar to that of the bedrock-dominated topography immediately to the south in Brown County. Thequadrangle is underlain entirely by Mississippian Borden Group bedrock. The Wisconsin Episode maximum limit is locatedless than one mile south of the southern boundary of the quadrangle. Radiocarbon ages from an exposure along Lick Creeksouth of Trafalgar indicate the Laurentide Ice Sheet reached its maximum extent just after 23,700 years ago. A second glaciallimit, marked by the Crawfordsville Moraine, crosses the quadrangle from northwest to southeast. TheCrawfordsville Moraine represents the limit of a readvance which occurred 21,700 years ago. This preliminary geologic mapis an interim geologic map product that documents progress in mapping the Quaternary geology of Johnson County,Indiana. The lithologic classification of the Quaternary units of Indiana was used in assigning map units.

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