Category Archives: #geoblog

Notes from the field: Mammoth Spring Quadrangle

DSCN2991Geologist Bill Prior inspects a small tufa dam near Riverside Resort on the Spring River, Fulton County, AR.

Geologic mapping of the Mammoth Spring 7.5 minute quadrangle was recently completed by the Arkansas Geological Survey’s STATEMAP field team. This project was funded in part by a grant from the National Cooperative Geologic Mapping Program through the United States Geological Survey. This is the 26th year of STATEMAP projects in Arkansas and represents an unprecedented effort in gathering geologic data within the state. STATEMAP was established in 1992 by a congressional act to encourage 1:24,000 scale mapping of geology by state geological surveys. Arkansas first participated in the STATEMAP program in 1994, and since then, the Survey has overseen the completion of eighty one, 1:24,000 scale geologic maps, including the Mammoth Spring quadrangle.

This year’s grant enabled the AGS to hire a geologist and partially cover field work expenses. Garrett Hatzell and staff geologists, Scott Ausbrooks and Bill Prior, worked together during the field data collection and map production phases of the project. This team approach has worked well for AGS mapping projects in the past, both for safety and efficiency.

The goal of STATEMAP is to classify and map rocks exposed at the surface into recognizable units, such as formations and members. Commonly, bedrock crops out along the sides or in the bottoms of creeks and rivers. For this reason, data collection in the major drainages is a priority. In areas where bedrock is covered, information may be inferred from data gathered at nearby outcrops. Since geologic formations are laterally extensive, disparate points taken on similar contacts can be connected across the mapping area to delineate each formation. Structural features, such as faults and folds, are also described and mapped.

Fieldwork for the mapping of Mammoth Spring quadrangle commenced on July 29, 2019 and continued until March 11, 2020 for a total of about 60 days spent in the field. From March through June, the geologists analyzed field data and prepared aspects of the geologic map. Formation contact lines and structural features were hand drawn on a topographic map base and were then digitally transcribed in ArcMap by Brian Kehner. The final layout of the map was completed in Adobe Illustrator by Garrett Hatzell, Angela Chandler, and Jerry Clark. Edits were provided by AGS staff geologists.

Printed copies of the Mammoth Spring geologic map (and many others) are available at the AGS office in Little Rock for $12.50, and digital copies may be downloaded free of charge as .pdfs from our website.

Here is the link to the recently completed Mammoth Spring quadrangle map: maps/24k/MammothSpring.pdf


Following a fatal accident at Saddler Falls on the Spring River in June of 2018, geologists from the Arkansas Geological Survey were called on to investigate a reported sinkhole that had caused the tragedy. Upon investigating the site, geologists observed that the falls present at this location were a series of tufa dams that spanned the river. Tufa, a type of limestone, is formed by the precipitation of calcium carbonate out of ambient temperature alkaline water. At Saddler Falls, a section of one of these dams had undergone a blowout of sorts at its base. Water began draining through this new opening at the base of the dam rather than flowing over the top as usual. This formed a dangerous whirlpool just upriver from the tufa dam structure. This new pathway was confirmed by performing a fluorescein dye trace at the whirlpool on June 25, 2018. After being poured into the whirlpool, the dye emerged about 15 feet away, just downstream of the tufa dam.

whirlpoolWhirlpool at Saddler Falls where a fatal accident occurred on June 9, 2018. Image courtesy of Arkansas Game and Fish Commission.

dye_traceDrone image of fluorescein dye trace (green) at whirlpool in Spring River near Saddler Falls on June 25, 2018. Dye emerged just below tufa dam after entering whirlpool. Image courtesy of BrainStorm Media.

The whirlpool hazard was mitigated on July 12, 2018 by using an excavator to remove the upper portion of the tufa dam, above the conduit, allowing water to flow freely through the dam. Subsequently, this area has become one of the most easily navigable areas of the many tufa dams along the Spring River.

tufadozerDestruction of tufa dam/whirlpool at Saddler Falls on July 12, 2018 by an excavator operated by Arkansas Game and Fish Commission. Image courtesy of Arkansas Game and Fish Commission.

remediated_whirlpoolArea after whirlpool was destroyed. Image courtesy of Arkansas Game and Fish Commission.

dry_tufa_damExposed tufa dam at Saddler Falls where whirlpool had formed.

tufa_dam_bottomThe underside of a tufa dam exposed by the excavator during mitigation of the Saddler Falls whirlpool.

The Spring River is a popular recreational river, attracting tens of thousands of tourists annually. After participating in the whirlpool mitigation effort and further observations of the tufa dams by AGS staff, it was determined that the Mammoth Spring quadrangle was of high importance for geologic mapping.

Tufa dams form on spring fed river systems around the world but had not yet received any thorough investigation in the Ozarks. Other large spring fed drainages in the area, such as Big Spring and the Current River in Missouri, do not produce tufa dams. Over 30 of these structures were documented and mapped on the Mammoth Spring quadrangle, and they continue to form downriver, off the quad, for many more miles. Small areas of tufa precipitation were also found on tributaries of the Spring River throughout the mapping area. Geologists from Missouri State University have also begun conducting studies of these tufa structures, which includes the coring of some of the tufa dams. The continued cooperation between the AGS and Missouri State University on this endeavor will hopefully lead to a better understanding of the formation and degradation of the tufa structures along the Spring River.

tufa_thin_sectionThin section of tufa dam material. Crumbly, porous calcite has loosely cemented chert and other alluvial gravels together, along with organic debris, to form the tufa dams. Image taken at 50x magnification with gypsum plate inserted for contrast.

MOstate_tufa_coringGeologists from Missouri State University coring a tufa dam near Riverfront Resort on the Spring River.

tufa_coating_ssTufa growth coating a sandstone cobble found in English Creek.

The Mammoth Spring quadrangle abuts the northern border of Arkansas, and lies within the Salem Plateau. The Salem Plateau is the oldest and most interiorly located of three broad plateaus that flank the Ozark Dome, centered in southeast Missouri, and together make up the Ozark Plateaus. Some 450 feet of Ordovician age carbonate and clastic rocks are exposed in the mapping area. These rocks formed from sediment deposited/precipitated in shallow epeiric seas which covered the area during that time. Only two formations were found to crop out within the mapping area: the Jefferson City Formation and the Cotter Formation.

Spring_River_raftingSTATEMAP team rafting down the Spring River.  

English_creekSTATEMAP team hiking/wading English Creek.

The Mammoth Spring quadrangle is drained by the Spring River and its tributaries. The largest and most extensive of these tributaries are Myatt and English Creeks. Geologic data was gathered along roughly fourteen miles of the Spring River which was traversed by the STATEMAP team via an inflatable raft. Both Myatt and English Creeks were hiked and waded along with numerous other smaller tributaries of the Spring River. Dolostone of the Cotter Formation is extensively exposed along the Spring River and its tributaries, across the vast majority of the quadrangle.

The Cotter consists of mostly gray-tan dolostone with interbedded chert nodules and small, sometimes oolitic, brecciated or conglomeratic chert layers, and sandstone lenses. Discontinuous sandstone lenses are increasingly abundant higher in the stratigraphic section. Mammoth Spring, in the northeast corner of the quadrangle, flows to the surface at almost 10 million gallons per hour. This flow travels for about 400 yards before joining the Warm Fork of the Spring River and continues southward. Near Mammoth Spring, the older Jefferson City Formation crops out as a 6-8 feet thick bench of conglomeratic/brecciated chert known as the Rockaway Conglomerate. Faulting to the south of Mammoth Spring, and a gentle southwesterly regional dip, take the Jefferson City into the subsurface as one travels south across the area.

Rockaway_boulderGeologist Bill Prior measuring a large boulder of the Rockaway Conglomerate near Old Town Spring in Mammoth Spring, AR.

Rockaway_outcropGeologist Scott Ausbrooks measuring the strike and dip of a steeply dipping exposure of the Rockaway Conglomerate, located 350 feet west of the Mammoth Spring.

If you have ever floated the Spring River, you would have almost undoubtedly noticed the busy BNSF railway line that runs along its east side. You likely would have also noticed the scenic natural rock exposures and the cuts created for the installation of this railway. The STATEMAP team was allowed the courtesy of a ride on the BNSF railroad in a hi-rail pickup truck to observe and record geologic data along the tracks. This led to the discovery and mapping of several exposed faults that otherwise would not have been recognized because weathering has covered them elsewhere.

rrnotesGeologists Scott Ausbrooks and Bill Prior taking notes while observing rock exposures along the BNSF railway which parallels the course of the Spring River through the mapping area.

rrcropOutcrop of Cotter Formation exposed along BNSF railway, adjacent to Spring River.

RRfaultFaulting in a Cotter Formation exposure along the BNSF Railway.

In addition to mapping, water samples taken from Mammoth Spring, the Spring River, the Warm Fork, and other nearby springs, were analyzed for a geochemical study of the area’s aquifers and surface waters. Characteristics such as conductivity and pH were measured in the field, while other chemical analyses were performed by the Laboratory Division of the Department of Energy and Environment in North Little Rock.

water_samplingWater sampling at Mammoth Spring, performed by STATEMAP team.

fracturedCotterHighly fractured dolostone bedrock of the Cotter Formation exposed in English Creek.

Special thanks to Dr. Doug Gouzie and his graduate students from Missouri State University for their involvement in the ongoing study of the tufa dams. Also a special thanks to Angela Chandler for writing the proposal and serving as this project’s Principal Investigator. A very special thanks to BNSF Railway for allowing us to ride along with them and examine all the outcrops we wanted to see. A special thanks to the staff of the Laboratory Division of the Arkansas Department of Energy and Environment for performing the chemical analyses on our water samples. Thanks also to all the private landowners who graciously allowed us access to their property.

Next year, STATEMAP will continue mapping adjacent to this area. The Stuart quadrangle, south of the Mammoth Spring quadrangle, will be the site of the next geologic mapping effort.

Geo-pic of the week: A lively sea in North Arkansas


Starfish pit 2Starfish pit 1

Starfish pit 4Starfish pit 6

Starfish pit 5Starfish pit 3     

The above pictures are of sandstone beds from a quarry in North Arkansas developed in a rock formation called the Batesville Sandstone.  Though it formed in a marine setting, the Batesville is typically composed of fairly homogeneous, flat-bedded rock with little evidence of inhabitation.  This spot is an exception.  The pictures clearly indicate the depositional environment was teaming with sea life at the time the sediment was emplaced.  

The abundant trace fossils, which preserve the activity of organisms rather than their physical form, show a variety of behaviors common to marine invertebrate animals that lived in Arkansas more than 330 million years ago.  Remnants of grazing traces of various snail and worm-like critters (A), resting traces (starfish; B), Dwelling burrows (sea-anemone or bivalve?; C), and locomotion trails (D) are indicators of the conditions present in North Arkansas near the end of Mississippian time.