HIGHLIGHTS OF THE UNIQUE GEOLOGY OF THE FORT DODGE, IOWA, AREA ___________________________________________________ Ryan J. Clark Geological Society of Iowa ______________________________________ April 12, 2014 Guidebook 92 Cover Photograph: Exposure along the north wall of the US Gypsum Company’s Flintkote quarry. Note the slumping of overlying Wisconsin Des Moines Lobe till into the solutionally-enlarged fractures of the underlying gypsum of the Upper Jurassic Fort Dodge Formation. (Photo taken by Ryan J. Clark on 3/18/2014 and used with permission by the US Gypsum Company.) Geological Society of Iowa HIGHLIGHTS OF THE UNIQUE GEOLOGY OF THE FORT DODGE, IOWA, AREA Author / Field Trip Leader Ryan J. Clark University of Iowa Iowa Geological Survey Iowa City, IA 52242-1319 [email protected] with contributions by: Huaibao Liu Raymond R. Anderson University of Iowa University of Iowa (Adjunct Asst. Professor) Iowa Geological Survey Iowa City, IA 52242-1319 Iowa City, IA 52242-1319 [email protected] [email protected] Robert R. Cody Robert M. McKay Department of Geological and Atmospheric Sciences Iowa Department of Natural Resources Iowa State University (Emeritus Faculty) Iowa Geological and Water Survey (Retired) Ames, IA 50011 Iowa City, IA 52242-1319 [email protected] [email protected] April 12, 2014 Geological Society of Iowa Guidebook 92 Guidebook 92 Acknowledgements We extend special thanks to the United States Gypsum Company and Mr. Matt Benzie, quarry/mill man- ager, at the Fort Dodge, Iowa, US Gypsum facility for allowing us to access their facility when no one else would. My gratitude to Robert D. Cody, Raymond R. Anderson, and Robert M. McKay for their splendid work on the GSB Guidebook No. 19 (1996) and the GSI Guidebook No. 67 (1999), much of which was used in the completion of this guidebook, my first as a member of the Iowa Geological Sur- vey. Finally, thanks to Richard Langel, Stephanie Surine, and Huaibao Liu of the Iowa Geological Sur- vey and Raymond Anderson for providing editorial support. ii Geological Society of Iowa TABLE OF CONTENTS Introduction by Ryan J. Clark ................................................................................................................ 1 Stop 1: Soldier Creek Member Exposure – Soldier Creek Nature Trail / 5th Avenue North .................................................................................................... 4 Stop 2: Fort Dodge Fault Zone and Mississippian Bedrock Exposure – Soldier Creek Nature Trail / Skateboard Park ....................................................... 7 Stop 3: United States Gypsum Company ......................................................................... 9 References ................................................................................................................................... 16 iii Geological Society of Iowa iv Geological Society of Iowa INTRODUCTION by Ryan J. Clark Iowa Geological Survey Iowa City, Iowa 52242-1319 [email protected] The Iowa Academy of Science annual meeting is at Iowa Central Community College in Fort Dodge, Iowa. Fortunately for the members of the Geological Society of Iowa (GSI), the Fort Dodge area hosts some of the most unique bedrock geology in the state; such as the basal limestone-clast conglomerates, economic gypsum beds, and desert red beds of the Jurassic Fort Dodge Formation. Previous geological field trips were centered in and around Fort Dodge in 1996 (Cody, et al., 1996, Geological Survey Bureau – Guidebook Series No. 19) and in 1999 (Anderson, et al., 1999, Geological Society of Iowa – Guidebook No. 67). Both trips visited the exposures along Soldier Creek that we will visit today as well as several area quarries. Of the many things that have changed in the last 15 years or so, safety awareness in quarries has increased dramatically, partly due to the increased enforcement actions by the Mining Safety and Health Administration (MSHA). This, of course, is a much needed improvement to the mining industry; however, it seems to have a negative effect on the geology enthusiast industry. Access to quarries, whether for geologic mapping conducted by Iowa Geological Survey staff or for weekend fossil hunters trying to inflate their collections, is becoming increasingly difficult. Quarries simply cannot afford the risk. That is why I am delighted that one quarry in Fort Dodge allowed us access for the purposes of this field trip. We are very lucky. The other thing that has changed over the last 15 years is the exposures themselves. For example, both of the previous field trips visited Snell Crawford Park, located along Soldier Creek. Just across the creek from a parking lot stood a ledge of gypsum about six feet (2 m) thick just above the creek level, with another 35 feet (10 m) of Fort Dodge Fm. red beds above it. Today, only three blocks of gypsum survive at creek level with a steep slope of dark red mud above it (Fig. 1). Needless to say, with the limited time we have for our trip, we will not be wasting any of it looking at the Snell Crawford Park section. Figure 1. Snell Crawford Park section in 1999 (left) and 2014 (right). Note the white gypsum bedrock exposed just above creek level. Much of the gypsum ledge has slumped and been buried over the past 15 years. 1 Guidebook 92 The rocks of the Fort Dodge Formation (Upper Jurassic) are the most intensely quarried bedrock units in Iowa. The formation includes a locally-occurring basal conglomerate (Shady Oak Member) and an overlying dark gray claystone, followed by a thick gypsum unit (Gypsum Creek Gypsum Member), and an upper suite of sandstones and mudstones called the Soldier Creek Member (Fig. 2). The formation has an extremely restricted areal extent, with its only known occurrences in central Webster County, Iowa. The thick and exceptionally high quality gypsum unit that dominates the formation has been mined for a variety of economic products since the mid-1800's. Today we will not see the Shady Oak Member since its natural outcrops are very poor and we were not allowed access to the only other exposures in the Georgia Pacific quarry on the west bank of the Des Moines River, but we will examine the other two Fort Dodge Formation members, the Gypsum Creek Gypsum Member and the Soldier Creek Member. The formation was the target of several early geologic investigations in the late 1800's and early 1900's. While these early studies discussed many details of the lithology, structure, and economic features of these strata, especially the gypsum beds, they failed to adequately answer many basic questions, such as the age, depositional environments, and the mechanics of their preservation. The high quality gypsum in the formation has been intensely mined for over 125 years, making the Fort Dodge Formation the only bedrock unit in Iowa that could potentially be completely removed from the geologic record. Figure 2. Composite stratigraphic section of the Fort Dodge Formation, from Anderson (unpublished). 2 Geological Society of Iowa Figure 3. Map showing the vicinity of stops #1 and #2. The field trip will begin on the campus of Iowa Central Community College at parking lot L5 just south of the Bioscience and Health Science Building (see map on back cover). Drive out to Highway 169 and turn right (north) and continue to 2nd Avenue S. Turn right (east) on 2nd Avenue S and continue to N 3rd Street. Turn left (north) on N 3rd Street then turn right (east) on Haskell Street. Then turn left (north) on N 5th Street. Once you get to 5th Avenue N, turn left (west) and go around the building (Humes Distributing, Inc.) to the parking lot by the trail. We will assemble there and walk to stops #1 and #2. 3 Guidebook 92 Stop 1 – Soldier Creek Member Exposure – Soldier Creek Nature Trail / 5th Avenue North **Please do not attempt to cross the creek and do not attempt to climb on the exposure, it is very slippery and unstable!** The type section exposure of the Soldier Creek Member is located on the west bank of Soldier Creek (Fig. 4) approximately where; 5th Avenue North would intersect the creek, if it were extended west. The name “Soldier Creek Beds” was first applied to the units at this location by Zaskalicky (1956) in an unpublished report on the gypsum resources of the area prepared for the U.S. Gypsum Company. More recently, the name Soldier Creek Member was applied to the unit by Anderson (personal communication), however the earlier assignment of a similar name in Iowa, the Soldier Creek Shale Member of the Bern Limestone, Pennsylvanian Wabaunsee Group, may force this name to be changed. Figure 4. Soldier Creek Member exposed at the 5th Avenue North section on the west bank of Soldier Creek. Approximately 50 feet (15 m) of Soldier Creek Member red, tan, and green mudstones, claystones, and highly calcareous sandstones/siltstones are exposed at this section (Fig. 5). Thirteen beds were described and tentatively grouped into five informal units. Comparing the rocks at the 5th Avenue North section with the section at Snell Crawford Park (~ 0.5 miles up Soldier Creek) and a section at the Kohl Brewery (~ 0.4 miles down Soldier Creek) published by Wilder (1919, p. 137-138), an additional 10 to 13 feet (3 – 4 m) of Soldier Creek Member strata are present in the subsurface. This constitutes the entire Soldier 4