QUATERNARY GEOLOGY OF THE PALEOZOIC PLATEAU REGION OF NORTHEASTERN IOWA
INTRODUCTION
Pikes Peak State Park is situated along the Mississippi River in Clayton County. The physiographic region surrounding Pikes Peak State Park is markedly different from other landform regions of Iowa. Most notably are the steep-sided cliffs, bluffs, deeply entrenched stream valleys, and karst features. In contrast to other parts of the state where glacial cover dominates, the surficial character of this area is bedrock controlled. The scarcity of glacial deposits led to the original term “Driftless Area”, indicating that the area had never been glaciated. However, later studies disproved this idea and the term “Paleozoic Plateau” was applied (Prior, 1976). Many researchers still use the term Driftless Area in their descriptions due to the limited exposures of glacial materials in this region.
EARLY STUDIES OF NORTHEASTERN IOWA - THE “DRIFTLESS AREA”
The landscape region of northeast Iowa was originally termed the “Driftless Area” due to the belief that this area had never been glaciated. Later studies indicated this was not the case, and it was termed the Paleozoic Plateau (Prior, 1976). The first geological investigations of the area were completed during the 1840’s under the direction of David Dale Owen (in Calvin, 1894). In 1862, Whitney was the first to document a Driftless Area in Iowa, Wisconsin, Minnesota, and Illinois and produced a map depicting this region. Chamberlain (1883) and Chamberlain and Salisbury (1886) later used the western line of Whitney’s map in their mapping of the Driftless Area boundary in Iowa. They also noted a “pebbly border of earlier drift” in all except a few townships. Although the researchers recognized this drift as foreign material, they did not believe these materials had been deposited directly by the ice, and thought they were possibly ice-rafted debris or the result of floodwaters.
Numerous studies and maps were published for this region by the Iowa Geological Survey from 1892 through the early 1900’s. McGee (1891) and Calvin (1894, 1906) recognized upland glacial materials and granite boulders, but did not consider them to be “proper” drift. Other studies described glacial outwash materials and boulders of non-native origin, but they were not determined to be directly deposited by glaciers. Due to the lack of glacially deposited materials, the high relief, and the extensive bedrock exposures, all these investigations led to the same conclusion- that the northeastern region of Iowa had not been glaciated.
While working on a Master’s thesis, A. J. Williams documented 80 patches of glacial drift in what had previously been called the Driftless Area. Due to the upland position of the drift and the differences between these materials and the Kansan drift to the west, he believed these deposits were Nebraskan in age. Prior to the completion of his PhD in 1923, a field conference was held, and at its completion researchers generally agreed that the drift east of the Kansan border was in fact deposited by a glacier and that the Kansan occurs both in the valleys and on the uplands. Kay and Apfel (1928) later published Williams’ map showing the locations of upland drift in northeastern Iowa. From then on the area was mapped as Nebraskan and no truly driftless area was recognized in Iowa.
In 1966, Trowbridge published a summary of previous works and included additional data from his studies of the region. In all, Trowbridge documented more than 100 occurrences of glacial drift and determined that these materials are till and not outwash. Trowbridge’s research further supported the idea that areas in northeast Iowa previously considered “driftless” by researchers had been glaciated.
The term Driftless Area came into being and was commonly used as a term to describe the region of high relief, heavily dissected, bedrock-controlled landscape of northeastern Iowa. The original area designated as the Driftless Area was much smaller than the region of rugged topography and associated flora and fauna commonly referred to by natural scientists. Therefore it was incorporated into a much larger area than initially defined and termed the Paleozoic Plateau. Within this larger area, many remnants of glacial drift are identified (Williams, 1923; Trowbridge, 1966), making the terminology “Driftless Area” incorrect. The term Paleozoic Plateau (Prior, 1976) is a better description for this physiographic region and incorporates the much larger topographically and ecologically similar area referred to by natural scientists and biologists.
GENERAL DESCRIPTION OF THE PALEOZOIC PLATEAU
The Paleozoic Plateau region has distinct physiographic features that separate it from any of the other landform regions in Iowa. The boundaries of this landform region are defined along the southern and western margins with the change from a rugged, dissected, rock-controlled landscape to that of the gently rolling, lower relief landscapes of the Iowan Erosion Surface to the west and the Southern Iowa Drift Plain to the south.
The Quaternary deposits of the Paleozoic Plateau are characterized by loess covered patches of isolated glacial till. Generally this area is a bedrock controlled terrain with deeply entrenched valleys, karst topography, and an integrated drainage network. The Paleozoic Plateau is unique as the only region of Iowa where bedrock dominates the landscape. In all other regions of Iowa the landscape features are dominated by unconsolidated materials and landforms (or dissections of them) including glacial materials, loess, and alluvium.
The characteristic features of the Paleozoic Plateau are representative of deep dissection by streams through gently inclined Paleozoic rock units with varying resistance to erosion. These rocks range in age from 350 to 600 million years old and include formations from the Devonian, Silurian, Ordovician, and Cambrian. The rocks dip gently to the southwest, exposing progressively older Paleozoic rock units in the northeast corner of the state. The more resistant rock types (sandstones, carbonates) form cliffs and escarpments high on the landscape whereas the more easily weatherable shales have gentler slopes. This differential weathering creates a landscape reflecting the local bedrock. Topography is also controlled by extensive karst development in this area forming caves, sinkholes, springs, and subsurface caverns.
In addition to the karst and other erosional features, the regional landform characteristics are also controlled by river development. The Mississippi River and its tributary valleys contain well preserved terraces, older floodplain deposit remnants, and entrenched and hanging meanders. All of these features indicate the complexity of the alluvial history and river development associated with glacial melting and drainage diversions.
The Paleozoic Plateau region is characterized by an abundance of bedrock exposures, deep and narrow valleys, and limited glacial deposits. The steep slopes, bluffs, abundant rock outcrops, waterfalls and rapids, sinkholes, springs, and entrenched stream valleys form a unique physiographic setting. These characteristics combine to form an area of many diverse microclimates that support varied flora and fauna communities not represented elsewhere in the state.
QUATERNARY MATERIALS IN THE PALEOZOIC PLATEAU
Glacial Till and Loess Deposits
The Paleozoic Plateau region was glaciated multiple times during the Pre-Illinoian. Willman and Frye (1969) identified two tills in Iowa as well as glacial outwash on upland surfaces in the Driftless Area of Illinois. Knox (1982) also showed that Pre-Illinoian till is present in the Wisconsin portion of the area east of the Mississippi River and holds that although there are driftless areas in parts of Wisconsin, that Iowa does not have driftless areas. Additionally, large parts of the Driftless Area in Minnesota show evidence of glaciation as well. Based on work by Hallberg (1980a) it has been determined that two Pre-Illinoian till units, the Wolf Creek and the Alburnett Formations, occur in the Paleozoic Plateau of Iowa.
The younger Wolf Creek Formation cannot be directly dated in northeast Iowa, but based on other studies it is younger than 600 ka, and it is estimated to be about 500 ka indicating the last time glacial ice advanced into this area (Hallberg and Boellstorff, 1978; Lineback, 1979; Hallberg, 1980b). Stream erosion and hillslope development since the last glaciation has resulted in the removal of most of the glacial materials, except those high on the divides, and has produced the dissected landscape we see today (Hallberg et al., 1984).
Upland surfaces are mantled with 3 to 6 meters of Wisconsin age loess which have been radiocarbon dated at 25.3 ± 0.65 ka (Hallberg et al., 1978). The end of loess deposition in Iowa is considered to be about 14 ka (Ruhe, 1969) and this coverage may obscure other glacial deposits. On the primary stream divides, 4 to 6 meters of loess overlies well-drained paleosols developed on Pre-Illinoian tills. The paleosols are generally 1 to 2 meters thick, but locally may be up to 2 to 5 meters thick. These thicknesses and other features are typical of Late-Sangamon paleosols. Yarmouth-Sangamon paleosols are only locally preserved on the divides. The Late-Sangamon paleosol and surface may truncate the Pre-Illinoian till and descend onto the Paleozoic bedrock. (Hallberg et al., 1984)
Mississippi River
Although many early studies suggested that the landform features of the Paleozoic Plateau are very old, more recent research indicates that the modern drainage system and dissected landscape of this region occurred during the Pleistocene. The oldest valley remnants are buried by Pre-Illinoian tills and may be middle to early Pleistocene in age, although the time of incision is not well constrained. Evidence is derived from studies of the upland stratigraphy and erosion, karst systems, fluvial and terrace deposits of the stream valleys, and the dating of speleothems.
Knox and Attig (1988) studied the Bridgeport terrace in the lower Wisconsin River valley, Wisconsin. Paleomagnetic dating of the Bridgeport terrace sediments indicate that they are older than 730 ka. The valley would have had to already be entrenched by this time, indicating a minimum age for these deposits. Therefore, they believe that the Mississippi River between northeast Iowa and southwestern Wisconsin was deeply entrenched by Pre-Illinoian time.
Research summarized in Hallberg et al. (1984)
suggests that the Mississippi River and its tributaries are of middle
Pleistocene age (500 ka). The major
drainage lines were established by Late Sangamon time, however major stream incision
probably began prior to the Illinoian.
The Upper Mississippi River valley likely originated as an ice-marginal
stream during what had been referred to as the “Nebraskan” glaciation. Current
terminology would place this as Pre-Illinoian.
The relationship between karst deposits and Pre-Illinoian tills can also yield information regarding the landscape evolution of the Paleozoic Plateau area. Karst ages have been determined by radiometrically dating speleothems. Fifty speleothems in Minnesota and Iowa have been dated (Lively, 1983). A few dates range from 250 ka to greater than 350 ka, but the majority of the dates fall into three general groupings: 163-100 ka, 60-35 ka, and from 15 ka to present. Speleothem dates provide minimum ages on major valley downcutting, which lowered the piezometric surface and allowed speleothem growth in the vadose caves. Speleothem growth is episodic and partially controlled by climate.
Wisconsin time represented one of the main periods of valley entrenchment when bedrock-cored, cutoff meanders formed. During the formation of these deep valleys, periglacial activity formed colluvial slopes and karst features collapsed, creating a mantle of bedrock-derived rubble on the steep slopes of related valleys. Between 9 and 25 ka the stream valleys underwent a complex history of erosion and aggradation in response to changes in glacial drainage in the Mississippi River basin. The role of isostatic rebound on the process of stream incision in the area is not clearly understood.
During the past 25 ka in the upper Mississippi River valley, there have been four major episodes of alluvial activity (Knox, 1996). The period between 25 ka and 14 ka was characterized by large quantities of bedload sediment being transported by a braided stream system. This aggradation has been related to outburst floods from glacial lakes and normal meltwater discharge from the Wisconsin glacier. An island braided channel system developed between 14 ka and 9 ka as large discharges from outlet failures of proglacial lakes and sustained low sediment flows caused major downcutting. Modern Holocene climate and vegetation systems developed from 9 ka to approximately 150 to 200 years B.P. The upper Mississippi River returned to aggradation as Late Wisconsin age sediment in tributaries remobilized. Dominant processes during this period involved minor downcutting, channel migration, and the development of fluvial fans and deltas at the junction of tributaries. The fourth episode encompasses the time since European settlement when agricultural land use, channelization, and dam building have greatly impacted the upper Mississippi River.
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from
Anderson,
R.R. (ed.), 2000, The Natural History of Pikes Peak State Park, Clayton County,
Iowa: Geological Society of Iowa Guidebook 70, p. 37 - 41.