Department of Geology
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Other Contributions (2)
Fluvial terrace history, Kings Creek, Manhattan, Kansas part of Vignettes:Vignette Collection
Natural cutbank exposures along streams can provide information about the geomorphic history of the stream valley. In a fluvial system, extrinsic factors, such as climate, tectonics, or land use, can lead to changes in stream behavior. For instance, a change in climate, such as an increase in the intensity of rain storms, might lead to an increase in frequency and magnitude of floods, which could result in stream-channel erosion. Other changes in extrinsic factors could cause different stream responses. A careful investigation of the landforms and deposits of a stream would allow a reconstruction of the history of changes in extrinsic factors. This vignette describes an example of such a reconstruction. Kings Creek, a tributary of the Kansas River in northeastern Kansas (Fig. 1), has experienced periods of aggradation (deposition) and degradation (erosion) on its valley floor. A cutbank exposure along Kings Creek (Fig. 2) on the Konza Prairie Biological Station permits a reconstruction of the Holocene history (the last 11,500 years) of the stream valley. The cutbank exposes the alluvium (stream deposits) underlying two stream terraces (flat surfaces marking former floodplains), which are separated by a terrace scarp (the steep slope between adjacent terraces) that is as much as two meters high in some places. The older of the two fills is composed of silty, tan-colored sediment that is poorly layered and contains little gravel. It underlies a terrace in the lower parts of Kings Creek valley whose surface slopes gently toward the valley axis from the surrounding bedrock hills. A thin lens of charcoal buried in the silty sediments yielded a radiocarbon age of 8920 years before present (Fig. 3); the estimated calendar age of this charcoal is 10,000 years before present. A moderately developed soil profile in the silty alluvial fill at the terrace surface suggests a period of non-deposition that lasted thousands of years. The lower of the two terraces is underlain by darker and coarser-grained (sandy) alluvium that contains some gravel in its lower part. The boundary between the two alluvial fills is abrupt, and can be clearly seen in the cutbank where the younger sandy alluvium rests on the older silty alluvium and abuts it beneath the terrace scarp (Fig. 3). This boundary is an unconformity – a surface between two bodies of sediment that marks a period of erosion or non-deposition. Charcoal collected from the gravelly part of the fill yielded a radiocarbon age of 1770 years; its calendar age is about 1700 years before present. From this and other information, the geomorphic history of this part of the Kings Creek valley can be reconstructed. During the last global glaciation, which culminated roughly 20,000 years ago, the Laurentide ice sheet did not extend far enough south to reach Kansas, but the Kansas climate at this time was cold and dry. At this time the valley of Kings Creek must have been scoured of any pre-existing alluvial fill because no sediments of this age have been found here. By about 10,000 years B.P. Kings Creek had begun depositing fine-grained sediment on its valley floor (aggradation A; Fig. 4). Much of this sediment may be reworked loess (wind-blown silt) from the surrounding hillsides, as suggested by its grain size (silty), its color (tan–the color of typical loess), and the surface of the sedimentary fill (which slopes toward the valley center from the hillsides, suggesting the source of the sediments was the valley walls). The valley floor continued to aggrade until sometime in the middle Holocene, when the stream changed its behavior and began downcutting to leave its former floodplain stranded as a terrace surface (terrace 1). Kings Creek downcut into the older silty alluvial fill to within a meter of the modern stream level. Although we don't have ages directly on the period of downcutting, it had to have occurred after the formation of terrace 1 and prior to the initial aggradation of the sandy alluvium (i.e., before 1700 yr B.P.). The first sediments laid down by the stream at this time were gravelly, and these sediments were gradually overlain by sandy alluvium, which filled the previous entrenched chute to within a couple of meters of terrace 1. Kings Creek abandoned its floodplain by downcutting at the end of the second period of aggradation, and formed terrace 2 (aggradation B, Fig. 4). The last period of stream downcutting was punctuated by brief episodes when a cut terrace (terrace 3 eroded into older deposits) and a small fill terrace (terrace 4) were formed (neither of which is visible in the photos shown here). The modern stream is entrenched as much as six meters below the surface of terrace 1 (Fig. 4). Why did Kings Creek change its behavior and shift back and forth between periods of aggradation and downcutting? Climate change is the most likely extrinsic factor in eastern Kansas over this time period that might cause the stream to aggrade or downcut its channel. Aggradation might have been coincident with times of relatively gentle rainfalls that didn't cause big floods of surface runoff, and downcutting may have occurred during periods when intense rain storms and large destructive floods were common. The fluvial geomorphic history of Kings Creek is similar to that of other small streams in the Great Plains, which reinforces the hypothesis that regional climate changes caused the changes in stream behavior, but the exact mechanisms have not yet been deciphered.
The Snowplow, Lake Bonneville, Utah part of Vignettes:Vignette Collection
Lake Bonneville was the largest of the late Pleistocene lakes in the Great Basin of western North America (Fig. 1). It occupied the basin of modern Great Salt Lake and was hydrographically closed during its transgressive and regressive phases (the time periods when the lake was rising, and falling, respectively, as shown in Fig. 2). The lake was hydrographically open, or overflowing, during its higher phases (>300 m deep at its maximum) (Fig. 2). The entire history of Lake Bonneville spanned more than 15,000 years. Well-preserved shorelines and deposits of Lake Bonneville are found throughout the lake basin. A prominent geomorphic feature called "the Snowplow" by G.K. Gilbert (1890) (Fig. 3) displays characteristics that are typical of many of the shorezone landforms of Lake Bonneville. The Snowplow is located in the central part of the Bonneville basin (Fig. 4). Gilbert noted the resemblance of the landform's shape to that of a snowplow, and thus gave it that name. In understanding the origin of the Snowplow it is important to remember that Lake Bonneville was hydrographically closed during its transgressive phase (which, in the altitude range of the Snowplow, lasted from roughly 21,000 to 18,000 years before present; Fig. 2). The lake reached its highest altitude and formed the Bonneville shoreline between about 18,000 and 17,400 yr B.P., then dropped 100 m catastrophically during the Bonneville flood (O'Connor, 1993; it was a tremendous flood along the Snake and Columbia Rivers, downstream from Lake Bonneville – within the lake basin, all the water between the Bonneville and Provo shorelines rushed out of the basin very quickly, maybe within a month, and lake level fell rapidly). After the flood the lake stabilized at a bedrock threshold (Red Rock Pass; Fig. 4), and continued to overflow until about 14,500 yr B.P., while it formed the Provo shoreline. In post-Provo time, the climate became drier in the basin and the lake dropped below its overflow threshold. The Snowplow consists of a stack of V-shaped gravel barriers, most of which were deposited during the transgressive phase of the lake. The lower-altitude V-shaped barriers are older than the higher ones. Gilbert referred to V-shaped barriers as "V-bars," and speculated that they were typical products of shorezone depositional processes in closed-basin lakes in the Great Basin. He did not know of any examples of modern V-bars, but since the 1890s, coastal geomorphologists have referred to similar features in marine and lacustrine settings as "cuspate forelands." In the case of the Snowplow and many similar features, the name V-bar is probably not appropriate in modern usage because a bar is a feature that forms in a submerged offshore setting, whereas a barrier crest is higher than the mean water level. The "V-bars" in Lake Bonneville are probably all barriers rather than true bars, so we refer to them as "V-shaped barriers" to preserve some of Gilbert's original terminology. Study the maps and images provided with this vignette (Fig. 5) to identify the V-shaped barriers produced at different lake levels during the lake's transgressive phase. The exact mechanisms involved in the formation of the V shapes are not fully understood, although it is clear that longshore transport of gravel was involved (longshore, or along-shore, currents are set up in the wave zone by waves striking a coast at an angle, and the currents are strong enough to transport gravel). In a closed-basin lake the water level is not held constant, so that if the lake level were to rise at a rate of several meters per year, and longshore currents were transporting large volumes of gravel from adjacent alluvial fans, the depositional landforms would be barriers or spits that were higher (measured from the base of the gravel to the crest of the barrier) than similar features formed in lakes with stable water levels. One of the major unresolved questions in the case of the Snowplow is whether converging longshore currents, from the south and from the north, might have produced the V-shaped barriers. In addition, the cause of the deflection of the longshore current or currents away from the coastline to produce the "V" forms at the Snowplow is not known, but whatever the mechanism was, it was effective at all altitudes higher than the Provo shoreline. How does the Provo shoreline fit into the story of the Snowplow? From basin-wide studies of Lake Bonneville it is clear that the Provo shoreline formed after the Bonneville shoreline had formed. Because of the rapid drawdown of the lake during the Bonneville flood, there was insufficient time to allow shoreline formation, comparable to the transgressive-phase shorelines in this area, in the vertical interval between the Bonneville shoreline and the Provo shoreline. The Provo shoreline platform is cut into the stack of V-shaped barriers that had been deposited during the lake's transgressive phase. Future field work at the Snowplow might help answer the question of the direction(s) of longshore transport. Field investigations could include ground-penetrating radar surveys to determine the orientation of foreset bedding, and therefore the direction of longshore transport, in different parts of the barrier complex. In addition, the lithologies of gravel clasts would provide clues to the provenance of the gravel and direction of transport, and grain-size analyzes might give evidence of the strength of currents and/or storms. Google Earth® could be used to study the Snowplow and other shorezone landforms of Lake Bonneville (for instance, take a look at the unnamed V-shaped barrier at the Bonneville shoreline just north of the Snowplow; 39° 54' 40" N. Lat.; 112° 46' 45" W. Long.).