Download The Mehrten Formation

The Mehrten Formation
Authored by: Steve Claar, Diana Dragomir, Matthew Hartman, Dakota Keene, Theron Sowers, and Leslie Williams
Study Areas
Lithologies
Image 7 shows the breccia that is the uppermost member
of the Mehrten Formation. The breccia is a matrixsupported breccia composed entirely of volcanic rock fragments. This bed is compositionally similar to the lahar. It is
this member that caps the formation. As a result, the ancestral topography has been inverted and what were once
valleys are now the resistant ridges in the Sacramento Valley area.
Image 7: Breccia (not exposed at stop 1.
Stop 1: Mehrten Formation
outcrop in road-cut on Sierra
College Boulevard in Rocklin.
Image 6 shows the upper exposed conglomerate. Clasts are identical to the lower
conglomerate. The matrix is compositionally similar, however slightly more iron red
(as evidenced by the red color). The uppermost exposed conglomerate is a matrixsupported rhyolitic-rich 3 meter thick bed. This unit is 50% grains and 50% matrix. As
with the lower-most exposed conglomerate bed, clasts are volcanic and metamorphic in origin. Matrix is coarse grained. Rock fragments are well-rounded but slightly
smaller than the lower conglomerate bed, representing a different depositional environment.
Stop 2: Mehrten Formation outcrop in road-cut on
Indian Hill Road in Auburn.
Image 5: Bedding boundary.
Image 6: Highest exposed congloomerate
Geologic map of study areas. (Wagner, Jennings, Bedrossian, & Bortugno,
1981). The yellow star is stop 1. The green star is stop 2.
Image 4 shows a typical interbedded lahar within the Mehrten Formation. Directly
overlying the conglomerate bed is a matrix-supported breccia composed entirely
of volcanic rock fragments. This 2 meter thick bed is 60% grains and 40% matrix.
The matrix is grey, coarse silt-sized, and easily broken by mechanical means. Volcanic rock fragments are poorly sorted, sub-angular, and display porphyritic textures. Commonly occurring rock fragments include andesite and rhyolite, with
trace amounts of red scoria. Grain size for these volcanic clasts range from granule
to pebble. The upper contact between the conglomerate bed and the lahar bed is
sharp and well-defined.
Stratigraphy and Regional Correlation
Image 4: Volcanic mud-flow (lahar)
Image 3: Sand lens.
The Mehrten Formation was formally defined in 1939 by A. M. Piper. The type section is located just north of the
“Mehrten Dam” (since renamed the Camanche Dam) along “Clements-Camanche Road” (since renamed Camanche Parkway North) in the town of Wallace, in Calaveras County. The type section outcrops in bluffs near
the ancestral bed of the Mokelumne River.
Image 2: Clast
The formation is Miocene to Pliocene in age and stratigraphically underlies the Laguna Formation (Piper, Gale,
Thomas, & Robinson, 1939; Shlemon, Horner, & Florsheim, 2000) . The Mehrten Formation was originally believed to unconformably overlie the Valley Springs Formation (Piper, Gale, Thomas, & Robinson, 1939) , however
in study areas other than the type section this unconformity has not been observed (Shlemon, Horner, & Florsheim, 2000) .
Image 1: Lower Conglomerate Bed.
Economic Value
The Mehrten Formation has been known as a well developed aquifer in the Sacramento region. The
water extracted from the Mehrten sediments is in good to excellent quality due to the highly permeable volcanic sands. This aquifer yields many good production wells near to us, well number 2 in
Citrus Heights Irrigation District is an unusually good producer that is entirely made from Mehrten
sediments. Reports have shown this well to produce 1,000 gallons per minute that has a drawdown
( the change in hydraulic head observed at a well in an aquifer, typically due to pumping a well as
part of an aquifer test or well test) of 10 feet. A nearly typical producing well is the Fair Oaks Irrigation District well number 2, it produces 1,440 gallons per minute that has a bigger drawdown of 31
feet. There are also cases of a well in the Mehrten Formation to produce 20 gallons per minute with
a drawdown of 40 feet ( well 9N/7E-12P2 just south of Folsom), this type of production can be
caused by the tuff-breccia that is impermeable and yields less water. (California Department of
Water Resources, 2016)
Image 1 shows the typical conglomerate of
the Mehrten formation. The lowest exposed
conglomerate bed is a grain-supported 2 meter thick unit containing poorly-sorted clasts
of volcanic, plutonic, and metamorphic rock
fragments. Cobble-sized andesitic rocks
(Image 2) make up the bulk of the composition. Freshly broken surfaces revealed hornblende and plagioclase phenocrysts. Outcrops
along the southern portion of the Mehrten
Formation consist of porphyritic andesitic and
rhyolitic clasts. Smaller percentages of plutonic and metamorphic rock fragments were noted. Within the conglomerate, small sand
lenses (< 1m in height and <2m in length) are
present (Image 3). Bedding boundaries are
gradational.
References
California Department of Water Resources. (2016, April 15). Water Well Standards. Retrieved from California Department of Water Resources: http://www.water.ca.gov/groundwater/well_info_and_other/
california_well_standards/b74-81chap1f.html
Gunther, R. L., & Schulmeister, M. (2005, May 1). Academic Emporia. Retrieved from Modesto Subbasin: A California
Aquifer Holding on its Own: http://academic.emporia.edu/schulmem/hydro/TERM%20PROJECTS/Gunther/
stanislaus.html
Piper, A. M., Gale, H. S., Thomas, H. E., & Robinson, T. W. (1939). Geology and Ground-Water Hydrology of the Mokelumne Area, California. Water Supply Paper, 61-71.
Shlemon, R., Horner, T., & Florsheim, J. (2000). Quaternary Geology of the Sacramento Area. Association of Engineering Geologists, Sacramento Section Guidebook for Field Triip.
Springhorn, S. T. (2005). Stratigraphic Analysis and Hydrogeologic Characterization of Cenozoic Strata in the Sacramento Valley Near the Sutter Buttes.
Stratigraphic correlation chart adapted from Springhorn, 2005.
Wagner, D. L., Jennings, C. W., Bedrossian, T. L., & Bortugno, E. J. (1981). Geologic Map of the Sacramento Quadrangle. Retrieved from State of California Department of Conservation: http://www.quake.ca.gov/gmaps/RGM/
sacramento/sacramento.html