Salt and Pepper Sands of Western Iowa
by Brian J. Witzke
The modern Missouri River drains a vast area of the American West, with many of its tributaries having headwaters in the Rocky Mountains. The Missouri forms Iowa's western border with Nebraska, and its channel has repeatedly shifted course across the broad north-south valley, which exceeds 15 miles wide in places. Unravelling the geologic evolution of this major valley to its present location has become an important element in the assessment of groundwater resources in western Iowa.
Reconstructing the ancestry of the Missouri drainage system involves various lines of geologic evidence. To establish a relative time framework, geologists must investigate the relationships between the various glacial-age deposits that lie above bedrock as well as the evidence of erosional gaps separating them. In general, these deposits consist of a complex sequence of glacial drift left by advances of continental ice sheets from the north at least eight times between about 500,000 and 2.5 million years ago. In addition, rivers deposited large amounts of sand and gravel during melting of these various glaciers as well as during warmer interglacial episodes like today. These buried deposits of sand and gravel are of particular importance to parts of western Iowa as sources of groundwater.
The Iowa Geological Survey initiated a study of these various river deposits in western Iowa during the late 1980's in an effort to understand their distribution, composition, origin, and water-bearing characteristics. Two general types were described: 1) sediments derived from glaciers or glacial deposits to the north and northeast, and 2) sediments resembling those in the modern Missouri and Platte rivers of Nebraska. Included in this latter group are fine-grained sediments called "salt and pepper" sands by well drillers in western Iowa. These sands form a potentially significant source of groundwater across some upland areas where they are commonly buried beneath 50 to 300 feet of glacial deposits. Scattered within the white quartz-rich sand grains are dark "pepper" grains which are identified as fragments of volcanic glass. Such volcanic grains are common in many modern and ancient river deposits known to have been derived from Rocky Mountain sources and deposited in eastward-flowing rivers.
No Rocky Mountain sediments are presently being deposited anywhere in Iowa because the Missouri River flows southward along Iowa's western border forming a barrier to eastward sediment transport. In western Iowa, tributaries of the Missouri River flow southwestward and contain sediment derived from the erosion of indigenous materials in Iowa, particularly wind-blown silt (loess) and older glacial deposits. However, western-derived sediments, especially the "salt and pepper" sands, are known to exist across broad areas of western Iowa. These sediments provide evidence of earlier deposition across Iowa by an eastward-flowing drainage system before the Missouri Valley existed in its present form and location. The modern Missouri River valley cuts across the complete sequence of glacial tills in western Iowa. This suggests that the formerly eastward-flowing drainage was diverted southward by successive glacial episodes and acquired its present position sometime after the last advance of glacial ice in the area about 500,000 years ago. The history of this older west-to-east drainage across Iowa has been difficult to determine, but some important clues exist.
The youngest of the western-derived sediments occur above the oldest glacial tills in western Iowa, suggesting deposition as recently as 1.2 million years ago. However, most "salt and pepper" sands occur beneath the full sequence of glacial deposits, suggesting that some, if not most, of these deposits pre-date glaciation in western Iowa (older than 2.5 million years). Comparisons with similar sediments in eastern Nebraska show the Iowa materials most closely resemble some deposits in Nebraska of early Pleistocene, Pliocene, and Miocene age (about 1.2 to 15 million years old). Of note is the close similarity with Ogallala Group deposits in Nebraska. The Ogallala is famous not only as a major aquifer on the Great Plains, but also as a significant source of abundant fossil mammal bones of late Miocene age (5 to 15 million years ago). Bones of small three-toed horses and rhinoceros are especially noteworthy in the Ogallala rocks of Nebraska.
Preliminary studies have characterized some general geologic relationships of these western-derived, "salt and pepper" sands in Iowa and have shed some light on the historical development of the Missouri River drainage; however, many questions remain. The full sequence of glacial, interglacial, and pre-glacial events has not yet been deciphered, even though western Iowa arguably preserves the most complete record of glaciation on the continent. In addition, the potential of these various sedimentary deposits to serve as aquifers is poorly understood. The need for dependable groundwater supplies in western Iowa is acute, and it is hoped that the Geological Survey will be able to conduct additional research in western Iowa that will provide more satisfactory answers to fundamental geologic and hydrogeologic questions in the area.
Adapted from Iowa Geology 1991, No. 16, Iowa Department of Natural Resources