The photo above shows trace fossils that record the travels of two trilobites. Trilobites are an extinct group of marine invertebrate animals, resembling horse-shoe crabs, that flourished for 100s of millions of years in the Paleozoic Era (540-250 mya). The tracks the animal left are known as the trace fossil, Cruziana. It appears that one traveled from the right side of the photo, the other from the left, until they met in the middle where they rested for a while. At the center of the photo are resting traces known as Rusophycus. Perhaps they became friends or maybe they were even more than friends? It is Valentine’s Day. Their traces are preserved in the Atoka Formation of west-central Arkansas.
Download a commemorative 25th anniversary STATEMAP Field Calendar here:
We are celebrating the 25th year of detailed geologic mapping in Arkansas made possible by the passage of the National Geologic Mapping Act of 1992. It established STATEMAP which distributes funds to the states, typically geological surveys, in the form of cooperative grants which are used to partially fund various geologic mapping projects. The first grant received by the Arkansas Geological Survey, then known as the Arkansas Geological Commission, was for a proposal in fiscal year 1994. Since that time, seventy-eight 1:24,000-scale geologic maps have been completed, with two more on the way this year. Two maps at the 1:100,000-scale have also been published. This marks an unprecedented commitment to gathering data about the surface of the earth in our state. Following is a factsheet summarizing the STATEMAP projects in Arkansas since 1994.
Here is the law establishing STATEMAP:
National Geologic Mapping Act of 1992
PUBLIC LAW 102-285
signed May 18, 1992
To enhance geologic mapping of the United States, and for other purposes.
Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled,
43 USC section 31a. Findings and purpose
The Congress finds and declares that–
(1) during the past 2 decades, the production of geologic maps has been drastically curtailed;
(2) geologic maps are the primary data base for virtually all applied and basic earth-science applications, including–
(A) exploration for and development of mineral, energy, and water resources:
(B) screening and characterizing sites for toxic and nuclear waste disposal;
(C) land use evaluation and planning for environmental protection;
(D) earthquake hazards reduction;
(E) predicting volcanic hazards;
(F) design and construction of infrastructure requirements such as utility lifelines, transportation corridors, and surface-water impoundments;
(G) reducing losses from landslides and other ground failures;
(H) mitigating effects of coastal and stream erosion;
(I) siting of critical facilities; and
(J) basic earth-science research;
(3) Federal agencies, State and local governments, private industry, and the general public depend on the information provided by geologic maps to determine the extent of potential environmental damage before embarking on projects that could lead to preventable, costly environmental problems or litigation;
(4) the combined capabilities of State, Federal, and academic groups to provide geologic mapping are not sufficient to meet the present and future needs of the United States for national security, environmental protection, and energy self-sufficiency of the Nation;
(5) States are willing to contribute 50 percent of the funding necessary to complete the mapping of the geology within the State;
(6) the lack of proper geologic maps has led to the poor design of such structures as dams and waste-disposal facilities;
(7) geologic maps have proven indispensable in the search for needed fossil-fuel and mineral resources; and
(8) a comprehensive nationwide program of geologic mapping is required in order to systematically build the Nation’s geologic-map data base at a pace that responds to increasing demand.
The purpose of sections 31a to 31h of this title is to expedite the production of a geologic-map data base for the Nation, to be located within the United States Geological Survey, which can be applied to land-use management, assessment, and utilization, conservation of natural resources, groundwater management, and environmental protection.
section 31c. Geologic mapping program
(c) Program objectives
The objectives of the geologic mapping program shall include–
(1) determining the Nation’s geologic framework through systematic development of geologic maps at scales appropriate to the geologic setting and the perceived applications, such maps to be contributed to the national geologic map data base;
(2) development of a complementary national geophysical-map data base, geochemical-map data base, and a geochronologic and paleontologic data base that provide value-added descriptive and interpretive information to the geologic-map data base;
(3) application of cost-effective mapping techniques that assemble, produce, translate and disseminate geologic-map information and that render such information of greater application and benefit to the public; and
(4) development of public awareness for the role and application of geologic-map information to the resolution of national issues of land use management.
(d) Program components
(3) A State geologic mapping component, whose objective shall be determining the geologic framework of areas that the State geological surveys determine to be vital to the economic, social, or scientific welfare of individual States. Mapping priorities shall be determined by multirepresentational State panels and shall be integrated with national priorities. Federal funding for the State component shall be matched on a one-to-one basis with non-Federal funds.
This rather handsome outcrop of the Wilcox group consists of alternating layers of sand and clay of the Eocene Epoch which lasted from about 56-34 million years ago. The Wilcox Group is a non-marine unit mostly composed of sand with lesser clay, silt, gravel, and lignite (low-grade coal).
This geologic unit is part of a larger sequence of loosely-consolidated sedimentary rocks exposed in south central Arkansas, south of Pulaski county. These rocks are the northern extent of the West Gulf Coastal Plain, a physiographic province that stretches from central Arkansas, south, to the Gulf of Mexico.
In the picture above, large black rectangular aegerine crystals are prominent in a rock type known as a pegmatite. Pegmatites are igneous rocks characterized by extremely large crystals. Sometimes they also contain unusual mineral species. This sample was collected from Magnet Cove, Arkansas. Magnet Cove, which is approximately 10 miles east of Hot Springs, is one of the few places in Arkansas where igneous rock is exposed at the surface.
Between 84 and 100 million years ago, magma was injected into the earth’s crust under central Arkansas where it slowly cooled and crystallized into igneous rock. Millions of years of erosion eventually unearthed that rock. Despite only being exposed over approximately 5 square miles, the rocks of Magnet Cove have yielded more than 100 different minerals. Rare minerals have been discovered there including a new variety of zirconium-rich garnet called Kimzeyite.
Why do rocks have beds? Are rock beds where geologists sleep? Sometimes, but that’s not the point of this article. The picture above, taken on the Goat Trail at Big Bluff, overlooking the Buffalo National River, is a great example of a sedimentary rock composed of many individual beds (layers). The reason that rocks are bedded is due to either gaps in deposition or abrupt changes in the grain size of sediment being deposited in an environment.
Here’s an example; when a storm causes a river to flood its valley, the water deposits sediment as the flood recedes. Typically, there’s a period of non-deposition before the next flood event deposits a new layer of sediment over that one. This time between floods allows weathering to alter the character of the first flood deposit. That weathered surface will eventually differentiate the flood deposits into distinct beds of rock.
Bedding can also form as a result of flowing water gaining or losing velocity. The size of sediment that water carries (and eventually deposits) is directly related to flow rate. A sudden change in flow rate creates bedding distinguished by differences in grain size.
A sinkhole is an area of ground that has no external surface drainage. Water that enters a sinkhole exits by draining into the subsurface. Many people are leery of sinkholes because of the damage they sometimes cause. Every now and then, a catastrophic sinkhole-collapse makes headlines, typically by swallowing someone’s house, or even draining an entire lake.
Not every kind of sinkhole is the dangerous kind though. The picture above shows a solution sinkhole. Unlike the feared collapse sinkhole, the solution sinkhole forms by chemical weathering of rock at the ground surface resulting in gradual lowering of the surface to form a depression. Solution sinkholes form in areas where fractures and joints in the bedrock create pathways through which rainwater can infiltrate the ground.
In, Arkansas, sinkholes are common in the northern part of the Ozark Plateaus where much of the bedrock is limestone or dolostone. These types of rocks are notorious for sinkhole development because they are soluble in weakly acidic rain water.
Like the White Cliffs of Dover, England, the “White Cliffs of Arkansas” (pictured above) are composed of chalk. Chalk is a marine sedimentary rock that forms of calcite-rich mud that accumulates in semi-deep marine environments. The mud is composed of the accumulated skeletal remains of algal microorganisms called coccolithophores. These algae grow and shed skeletal parts called coccoliths which they arrange around them, in life, in a structure called a coccosphere. Below is a scanning electron microscopic image of some coccospheres (borrowed from news.algaeworld.org).
Chalk in Arkansas is found in the Annona Formation, which formed in the late Cretaceous Period, and crops out in southwest Arkansas as well as parts of Texas. In addition to being mined to make blackboard chalk, this resource is also used in brick, and cement manufacture.