North Dakota Geological Survey Edward C. Murphy, State Geologist 24K: Amdn - sg Lynn D. Helms, Director Dept. of Mineral Resources Surface Geology

(J Amidon Quadrangle, North Dakota U ) N W IP S E M R U S L P U o ' " o ' " G R 103 22 30 103 15 00 A ) (ROCKY RIDGE SOUTH) R. 101 W. R. 100 W. (D 46o 30' 00" 46o 30' 00" Edward C. Murphy Qal Tsb Qw Qls Qal Tsb 2008

Q ls Qal EXPLANATION Qal SYSTEM Tsb Qls HOLOCENE

Tsb Mc Abandoned Coal Mine

Tsb Ef Engineered Fill

Qw (/85 HOLOCENE/

Qal Alluvium Qw Brownish gray to black sand, silt, clay, and lenses of gravel; floodplain deposits along recent drainages. Typically less than 50 feet. thick. Qal Qw Qw Qw Windblown Features (/85 Qw Most of the deposits mapped as Qw in this quadrangle are small semi-circular depressions or blowouts that were created when the wind scoured into poorly cemented sandstones that were exposed at the surface. These blowout features are softened where they have been repeatedly farmed, but are still quite evident on aerial photographs. The northeast corner of section 33 contains northwest-southeast trending longitudinal blowouts followed downwind (to the southeast) by a dune field. Qw Qal Qls Landslide Deposits Qw Variable mixture of strata and deposits that have slid to the base of steep slopes. Tsb Tbc Qw Qc Colluvium

Tbc Tsb Poorly sorted mixture of sand, silt, clay, and gravel forming aprons at the base Qal of steep slopes. Qal Qw Qal Tbc T Qw bc Ef Qp Pediment Gravel Qls Qls An apron of gravel that slopes away in all directions from the Chalky Buttes. Tbc Tbc The gravels were derived from the White River and Arikaree strata that Qal comprise the Chalky Buttes and are dominated by tuffaceous siltstones and M Qal c sandstones, marlstones, and reworked garvels from the Qc (primarily volcanic porphyries and quartz). These pediments range from a few

T. 1 35 N. Tbc Qls feet to tens of feet in thickness and are the major source of gravel in the area. T. 1 34 N. Tbc Qp1 Pediment Gravel Mc Qw Qp1 are gravels that lie on an older pediment surface that is generally preserved Qal on isolated buttes and ridges that are elevated above the younger, more Qal continuous Qp surface. These pediments range from a few feet to tens of feet in thickness and are a source of gravel in the area. Qls

Qal TERTIARY SYSTEM Qal Ef Mc Tsb Qp Ta ARIKAREE FORMATION Qp The Arikaree Formation is up to 125 feet thick in this area and consists of interbedded gray to green colored marlstone as well as tuffaceous siltstone,

) sandstone, and conglomerate. The sandstone ranges from poorly to very-well ) E E T K

T cemented. The marlstone caps the Chalky Buttes, is vuggy in places, A U Qal L

B Tbc contains conglomeratic lenses, pebbles, and chert nodules. The dip of the E K T I C marlstone blocks indicates that this unit was at one time stratigraphically H A Tbc L W ( B higher and is out of place. The Arikaree Formation unconformably overlies ( Qp Qp the White River . Tsb Qp Qal

Qp1 WHITE RIVER GROUP Qal Tb The Brule Formation is roughly 25 feet thick in the area and consists of inter- Qp bedded white to pink colored siltstone and mudstone. The rocks are generally

! Tsb tuffaceous, highly fractured, and weather to rounded or bulbous surfaces. ! The Brule Formation was removed by erosion prior to deposition of the Arikaree !

Qp1 Formation along the north edge of the Chalky Buttes. !

!

Tbc !

! Qls Qp Qls Qp Tc CHADRON FORMATION Qal Tsb

The Chadron Formation is the basal in the White River Group ! Qc and consists of two members; the Chalky Buttes Member and the overlying

! Tsb Qls South Heart Member. The Chalky Buttes Member is up to 80 feet thick and ! Tc Qls

!

! consists of grayish green to white conglomeratic, cross-bedded, poorly-

! Ta Tbc ! Qls Qp1 cemented sandstone. Pebbles include volcanic porphyries, quartz and some ! petrified wood. The South Heart Member is up to 100 feet and consists of

! Qls Qls Ta Qls brown, pink, and green colored claystone. The claystone is bentonitic with a ! Qp1 popcorn weathering surface and contains silicified zones.

! Qls

Qp ! Qls Qls PALEOCENE

! Qls

!

Tsb Qls Tsb SENTINEL BUTTE FORMATION ! Tc Qls The Sentinel Butte Formation is roughly 250 feet thick in this area and is ! Tsb unconformably overlain by the White River Group. The Sentinel Butte consists

! of interbedded brownish gray sandstone, siltstone, mudstone, and lignite. The

T Sentinel Butte rocks are bleached white to purple where they underlie the

b ! c Tbc Tsb White River unconformity. Ta ! Qls

! Qp Tb Tbc BULLION CREEK FORMATION T bc ! Qls The Bullion Creek Formation consists of interbedded, yellow to light gray sand- ! stone, siltstone, mudstone, and lignite. Because the lithologies are similar ! between the Bullion Creek and Sentinel Butte Formations, these units are ! Tb differentiated on color. The Bullion Creek is lighter and more brightly colored Qls

! than the drab-colored Sentinel Butte.

Qal ! Qp Tb Tsb HEALTH WARNING: The Arikaree and White River strata and the pediment gravels in ! this area may contain erionite. Erionite is a fibrous zeolite that has been linked to lung Tc ! cancer in other parts of the world. Until scientists can determine the risks of exposure to this mineral, precautions should be taken to avoid inhaling dust generated from these rocks.

Tbc ! Tc Qls

!

! Geologic Symbols Qls Tsb

!

! Tsb Qls Known contact between two geologic units Tbc Qls Qp Qls Qls ! Qls (85 al Qls

/ ! Q Qls Approximate contact between two geologic units

! Tc Tc Tsb ! Tsb Qls Ta ! ! ! ! Tc Tc Tb Contact between two buried geologic units Tbc Qls Qls Tb Qp

! Qls Qp1 (Sentinel Butte and Bullion Creek Formations) a

Qls T Qls ! ! Qls Qls Tb Other Features

! Tbc Qp ! Qls Ta Qls (/85 Federal Highway Unpaved Road

!

! Qls ! Qls Correlation of Map Units ! Qls Qls Qls Qp ! Ta Tb Ef Mc Qp1 y Qls r

! a Holocene Qp Tsb Qp Tc n Qal Qls

! r Qw Qc Qp Qal Tsb e ! Tsb ! Qp Tsb t Qp1 a o ' " Qal ! o ' "

46 22 30 46 22 30 u ( Pleistocene ) M E o ' " (AMIDON SE) o ' " I K N Q A 103 22 30 103 15 00 E L R T A R L S A P W R E I T o N S G ( S Scale 1:24,000 10 ) Miocene Ta MN

y Oligocene Tb r

0 0.5 1 a i t r

e Eocene Tc Miles T Tbc o ' " o ' " Lambert Conformal Conic Projection Standard Parallels 46 22 30 and 46 30 00 Paleocene 1927 North American Datum NGVD 1929 Tsb 2001 Magnetic North Amidon Quadrangle, North Dakota USGS 7.5 Minute Topographic Map Contour Interval 20 Feet Declination at Center of Sheet Road Layer Rectified to 2003 NAIP D igital Orthophoto This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program. Cartographic Compilation: Elroy L. Kadrmas