sandstone - Ogallala sandstone - hand specimen from the Ogallala aquifer

Description

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Medium-grained arkosic sandstone - teaching hand specimen of the Ogallala sandstone that forms the High Plains (Ogallala) aquifer
The Ogallala Formation forms the High Plains aquifer, supplying 30% of the groundwater used for irrigation in the U.S. As the Rocky Mountains underwent uplift in Miocene time about 10 million years ago, a thick wedge of clay, sand and gravel, sometimes called the "gangplank," spread eastward from the mountain front, once running continuously across Wyoming southward across eastern Colorado and into the Texas Panhandle where these specimens were collected. The early Ogallala was a series of braided-stream deposits that filled pre-Ogallala valleys under humid conditions. These were later covered by aeolian - wind blown - sand and silt, which were eventually capped by a layer of caliche, almost impervious, which impedes groundwater recharge in an already arid area.
The Ogallala and its caliche cover form the caprock of the Llano Estacado or "palisaded plain" of the Texas Panhandle. South of Amarillo, the Prairie Dog Town Branch of the Red River, in more humid times, cut through the Ogallala to expose Permian redbeds in Palo Duro Canyon, site of the star of Texas state parks. East of the state park, these rocks are exposed in road cuts where Texas Highway 207 crosses Palo Duro Canyon.
This sandstone is Plio-Miocene (4-6 million years) in age and is cemented with calcium carbonate, so it will effervesce with a drop of hydrochloric acid. It is composed primarily of grains of quartz and feldspar with flakes of muscovite mica visible in hand specimens. Arkoses are immature sandstones, with feldspar and without the rounding of the sediment grains that occurs with longer transport.  An interesting example.
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