Cenizo Journal Fall 2022 | Page 24

Northwest to southeast structural cross section after King ( 1937 ). Location of section is shown on Figure 2-7 . Blue units tend to be aquifers , while brown units tend to be aquitards . the Marathon Limestone is the most significant aquifer unit based on its ability to provide relatively large quantities of water . The town of Marathon obtains its water supply from wells completed in the Marathon Limestone .
Aquifers and aquitards ( areas where water can ’ t permeate the rock structures well ) that occur in the Marathon Aquifer region are the result of interactions of rock composition and the cumulative effects of a complex geologic history that has produced numerous fractures , faults , and folds . Lithology influences the mechanical properties of the rock , which affect the susceptibility of the rock to fracturing and determine whether the rock exhibits a brittle or ductile response to stress . Prior tectonic and structural events have resulted in the development of localized , welldeveloped fracture and joint permeability systems . Minor karstification ( cavities ) within the Marathon Limestone has also occurred , as confirmed by local well drillers .
Grouping formations by age and water well productivity is a useful starting point for conceptualizing the aquifer system , although it must be kept in mind that the stratigraphic units vary significantly , and some units may include both aquifer and aquitard intervals . For example , TWDB well records may indicate if water is produced from limestones of the upper or the lower Marathon Formation . Bentonites at the top and the mega-conglomerate in the middle of the Marathon Limestone are unlikely to produce water , but the layers of limestone in between can .
As part of the conceptual model development , a hydrostratigraphic framework was created to form the geometry for a future flow model . To examine the project data and test the geometry in three-dimensional space , a three-dimensional geologic model was created using the software package Leapfrog Works ® by Seequent . The three-dimensional geologic model is a digital representation of the hydrostratigraphic structure . Once the composition and number of layers were established , the Leapfrog model was constructed using a variety of data sources .
The lateral extent of the three-dimensional geologic model is the study area as defined by the Marathon Aquifer extent with an addition to the northeast of the town of Marathon , plus a 1 mile buffer around the aquifer boundaries . The vertical extent is defined from the topography to a
horizontal plane surface set at an elevation of 2,000 feet above mean sea level . This allows the computer model to predict what the layers of geology and water look like for approximately 2,000 feet below the surface .
A 30-meter digital elevation model grid obtained from the USGS National Map Seamless Server defines the land surface of the threedimensional geologic model . Surface geology maps originally surveyed by King in 1937 served as general guides for the interpretation of surface geology . The historical maps were scanned , georeferenced , and imported into a three-dimensional workspace , and then draped on top of the digital elevation model .
Using a combination of the surface geology maps and the Geologic Atlas of Texas shapefiles , the model surface geology was interpreted by comparing the two data sources and blending them together through hand-drawing polylines using drawing tools in Leapfrog . During this process , care was taken to preserve the major features while generalizing some of the finer detail . This was done while keeping in mind an interpretation of the subsurface structure . Defining the hierarchy of the three-dimensional geologic model surfaces ( i . e ., how the software will direct the surfaces to interact in space that result in digital volumetric shapes ) was also an important consideration in the construction of the model files .
To create the subsurface interpretations , 19 cross sections ( A – A ’ through S – S ’) from King ( 1937 ) were scanned , georeferenced , and imported into the three-dimensional workspace .
The majority of the existing recorded wells are completed in the alluvium , Tesnus Formation , and Marathon Limestone , which are aquifer units . However , there are also a lesser number of wells completed in what are generally considered to be aquitard units , such as the Gaptank and Haymond formations . This is not surprising , as the units may produce small amounts of water from fracture zones even though they are not as productive as some of the other formations . In the Marathon Aquifer area , as with many rural areas , landowners will often drill to the depth where they first encounter a source of water sufficient for their needs , and for local livestock or domestic use , relatively small well yields will suffice .

24 Cenizo Fall 2022