PROJ00132

• Contract/PO #: FS0014X
• Contractor Name: Southern High Plains Ground Water Management District
• Contract Type: Loan
• Water District: 0
• County: Prowers
• Loan Projects - Doc Type: Feasibility Study

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7.2.9 Greenhorn Limestone
The Greenhorn Limestone is mapped above the Graneros in the Bear Creek and Plum
Creek drainages in the northwestern part of the Basin. The Greenhorn is a light- to dark-
gray, thin-bedded, chalky limestone with shale interbeds. Both the Greenhorn and
Graneros pinch out to the southeast (i.e., the beds were eroded prior to the Tertiary
deposition of the Ogallala Formation). The Greenhorn was not distinguished or shown in
cross-sections as it is thin and shallow in this area.

7.2.10 Tertiary Volcanics
Intrusives:
Minor Tertiary-aged volcanics are present in the Basin and predate the Ogallala
Formation, which truncates some of the related volcanic dikes. The Two Buttes intrusive
is just north of the Baca County/District boundary in T. 27 N., R. 46 W. This intrusion has
lifted and exposed some of the deeper sediments and helps define the drainage divide.

Extrusives:
Several mesas in the southwestern part of the Basin are capped by basaltic lava flows.
The flow rocks range from 20 to over 100 feet thick and were deposited on top of the
Ogallala Formation. Thus, these extrusives are late Tertiary to Pliestocene in age and
chronologically should be listed after the Ogallala, but are noted here for continuity of
rock type.

7.2.11 Ogallala Formation
The Ogallala is a heterogeneous alluvial/colluvial deposit of unconsolidated sand, gravel,
silt and clay. It is locally bound by caliche or cemented with calcite or silica and contains
some freshwater limestones. The Ogallala once covered the entire Basin, but has been
eroded in major drainages to expose Cretaceous bedrock. The Ogallala ranges from 0
to 310 feet thick in the Basin and is the dominant member of the High Plains aquifer. It
thickens to the east and southeast into Kansas.

7.2.12 Quaternary Alluvium and Eolian Sediments
The alluvium includes stream deposits of silt, sand and gravel in and adjacent to major
stream valleys. The most significant alluvium is present in the Cimarron River valley in
the southeastern part of Baca County. Here alluvial deposits are reportedly up to 80 feet
thick. Some lesser alluvial sand and gravel occurs in the canyon bottoms and major
drainages in the southwestern part of the Basin.

VII-4