### Pleistocene and Holocene Landscape Development of the South Platte River Corridor, Northeastern Colorado

By: Margaret E. Berry**Janet L. Slate**Emily M. Taylor**

                            https://doi.org/10.3133/sir20195020

#### Links

-                                                                                                                             Document:                                                                                                                                                                                                                                                  **Report**                                                                                                     (12.2 MB pdf)                                                                                                                                     
- Related Works:                               - **Scientific Investigations Map 3331:**  Geologic map of the Orchard 7.5' quadrangle, Morgan County, Colorado
  - **Scientific Investigations Map 3344:**  Geologic map of the Masters 7.5′ quadrangle, Weld and Morgan Counties, Colorado
  - **Scientific Investigations Map 3396:**  Geologic map of the Weldona 7.5′ quadrangle, Morgan County, Colorado
  - **Scientific Investigations Map 3408:**  Geologic map of the Fort Morgan 7.5′ quadrangle, Morgan County, Colorado 
- Data Release:  **USGS data release**  - Data release of OSL, 14C, and U-series age data supporting geologic mapping along the South Platte River corridor in northeastern Colorado
- Download citation as:  RIS                         |                         Dublin                            Core

#### Abstract

First posted April 24, 2019

**For additional information, contact:**
Director, Geosciences and Environmental Change Science CenterU.S. Geological SurveyBox 25046, MS 980Denver, CO 80225

This report provides a synthesis of geologic mapping and geochronologic research along the South Platte River between the town of Masters and the city of Fort Morgan, northeastern Colorado. This work was undertaken to better understand landscape development along this part of the river corridor. The focus is on times of rapid change within the fluvial system that had a marked effect on the landscape. The study area is susceptible to drought, which destabilizes vegetation and makes the landscape vulnerable to eolian activity. This is reflected in a landscape that is largely covered by eolian sand and lesser amounts of loess. Past glaciation of the river’s headwaters had a major influence on river discharge and sediment supply, as have major flood events particularly on unglaciated tributaries heading on the piedmont.

In the mapping area, fluvial deposits of the South Platte River system span the Pliocene and early Pleistocene(?) deposits of Nussbaum Alluvium to present-day deposits of the active channel and floodplain. Results of the study indicate that along this stretch of the South Platte River, the early Pleistocene and first half of the middle Pleistocene were times of net incision, periodically interrupted by episodes of aggradation that resulted in deposition of alluvium that has been correlated to Rocky Flats Alluvium, Verdos Alluvium, and Slocum Alluvium. Net incision between depositional events formed a series of poorly preserved terrace deposits along the valley sides that are now largely covered by eolian deposits. Sometime after about 380 thousand years, the river cut a deep paleovalley into Upper Cretaceous Pierre Shale that was then filled with a thick sequence of inferred Louviers Alluvium (coeval with Bull Lake glaciation). Net aggradation continued during the late Pleistocene, resulting in burial of the Louviers paleovalley with a thick sequence of mainstream and sidestream Broadway Alluvium (coeval with Pinedale glaciation). Subsequent incision during the late Pleistocene–Holocene transition formed the Kersey (Broadway) terrace, whose riser forms a prominent bluff on the south side of the river valley. This episode of incision spanned a very short period and was followed by renewed aggradation that deposited the next-lower terrace alluvium (Kuner terrace alluvium). The Kuner terrace level was probably abandoned sometime around the beginning of the middle Holocene. Low terraces on the valley floor indicate that the river has been primarily cutting and backfilling laterally rather than incising during the late Holocene.

Synthesis of geologic mapping and chronologic data generated in this study indicate that the South Platte River in northeastern Colorado likely was highly sensitive to rapidly changing environmental conditions or crossed threshold conditions that triggered rapid geomorphic response during major climate changes associated with the late Pleistocene–Holocene transition. Historical times have been another period marked by rapid incision, reflected by gully incision and headward erosion in tributary valleys draining the north side of the South Platte River. This historical erosion could be related at least in part to extensive construction of irrigation ditches and reservoirs in the late 1800s–early 1900s, which altered drainage paths and groundwater flow and could have amplified natural factors such as climate change or intrinsic geomorphic instabilities within the system.

#### Suggested Citation

Berry, M.E., Slate, J.L., and Taylor, E.M., 2019, Pleistocene and Holocene landscape development of the South Platte River corridor, northeastern Colorado: U.S. Geological Survey Scientific Investigations Report 2019–5020, 22 p., https://doi.org/10.3133/sir20195020.

ISSN: 2328-0328 (online)

#### Study Area

#### Table of Contents

- Acknowledgments
- Abstract
- Introduction
- Late Pleistocene–Holocene Transition
- Late Holocene Terrace and Gully Formation
- Summary and Geomorphic Implications of River Stratigraphy
- References Cited

Part or all of this report is presented in Portable Document Format                        (PDF). For best results viewing and printing PDF documents, it is recommended that you download                        the documents to your computer and open them with Adobe Reader. PDF documents opened from your                        browser may not display or print as intended. Download the latest version of Adobe Reader, free of charge. More information                        about viewing, downloading, and printing report files can be found at the common download problems FAQ.