This beautiful sample was recently collected from Pulaski County and donated because of the rare crystal shapes of the minerals. This cavity, or vug, was found in an igneous dike in the Jackfork Formation.
The photos below are magnified to show the crystal shapes. Notice the shape of the white calcite crystals that resemble poker chips. These crystals grew differently because of the cooling rate of the mineral-rich solution. Multiple growth stages are shown by the stacking of the layers. On top of the calcite are pyrite crystals that look like strips of wire, hence the name wire-pyrite or filiform pyrite. The pyrite crystals probably grew very fast to produce this shape. Enjoy the photos and If you have an ultraviolet light, check for fluorescence because this calcite also glows red.
Map depicting the current status of geologic mapping along the White River near Batesville, Arkansas. The most detailed geology available for the area to the southeast is from the 1:500,000-scale Geologic Map of Arkansas (Haley et al., 1993). To the northwest, 1:24,000-scale geologic mapping done for STATEMAP and the Earth Mapping Resources Initiative over the past 25 years has superseded it. The difference in detail is obvious.
A geologic map depicts the known geology of a given area. The level of detail displayed on a that map is limited by the size of the area represented. The ratio between the area shown on the map and the actual surface of the earth is called scale. All maps must be drawn to scale because there is a limit to how big a map can be and still be useful. The smaller the scale, the larger the area depicted on the map; the larger the scale, the smaller the area represented. Maps of smaller areas can have more detail than those showing larger areas. In Arkansas, we are fortunate to have the geology of our entire state summarized on one map, the Geologic Map of Arkansas, found here: https://www.geology. arkansas.gov/docs/pdf/maps-and-data/geologic_maps/geologic-map-of-arkansas-1993-(34×52).pdf. This map was drawn at the 1:500,000 scale, meaning that one unit on the map represents 500,000 units on the ground. As an example, one inch on our state geologic map projects to almost 8 miles on the surface. Maps of the coterminous United States are often 1:1,000,000 or smaller, and those of the North American continent, 1:5,000,000.
Maps that cover less area but offer more detail are in high demand for any number of reasons and providing them has always been the main function of geological surveys. Since 1857, the geological survey of Arkansas, now called the Office of the State Geologist, has consistently improved the detail and extent of geologic mapping in our state. Currently, OSG geologists are in the field working on two separate geologic mapping projects, both cooperatively funded by the state and the US Geological Survey. The mappers are locating rock outcrops, describing them in detail, and recording the data they need to produce maps at the 1:24,000 scale. This scale, the same as that used on readily available 7.5-minute series topographic quadrangles, allows our geologists to depict divisions between rock units and any structural features present in much more detail than currently available. Once published, these maps will help land managers and others with decisions regarding critical resource development, feasibility of infrastructure projects, groundwater assessment, mineral and energy exploration, resource extraction, and the location of possible geohazards like karst- and landslide-prone areas.
One geologic mapping program, called STATEMAP, has fielded geologists for the past 28 years and is responsible for mapping 93 quadrangles at the 1:24,000 scale, an area of over 5,500 square miles. For the past six years, the Earth Mapping Resources Initiative has mapped areas adjacent to known occurrences of critical minerals. The first project focused on recently discovered rare earth elements around Batesville, and currently, projects are targeting areas in north Arkansas near historically mined deposits of lead and the critical mineral zinc.
The geology typically depicted on these maps is a hybrid of bedrock units and alluvial deposits. These units are symbolized by color according to their age of deposition. Bedrock units, typically formations and members, are laterally extensive and have discernable contacts with other units above and below. Here again, scale can be an important consideration as thinner units are difficult to display on small-scale maps. That’s why mapping at larger scales, like 1:24,000, is so useful. All but the thinnest units, those less than 20 feet thick, can be seen on maps of this scale and don’t have to be lumped together with other units as is often the case on smaller-scale maps.
Mapping at this scale requires much more than just a quick drive-by, but rather many weeks of hiking in often adverse conditions to find bedrock outcrops and gather the data needed to distinguish and draw contacts and structural features on a topographic base. Once drawn, the map must be digitized, and a layout started. A description of each rock unit, a cross-section, correlation chart, stratigraphic column, joint diagram, and symbol key are then created and added to the layout. When the mappers have everything finalized, they submit a draft copy to a rigorous review process involving our entire staff. After edits are made, the map is published and made available on our website. Producing geologic maps takes a full year, so progress is slow but steady. Typically, we publish two or three each year. And, despite our progress, there is still much to do and an ever-present drive for greater detail.
Office of the State Geologist Blog 