A preliminary report by Midland, Texas geologist and rock enthusiast Bill Halepeska. This report will be updated from time to time as Bill’s research continues.

A locality yielding particularly diverse and colorful materials has been documented after eight years of effort. Diversity includes color, structure/shape, and inclusions. The range in colors is noted to be particularly interesting, and includes various pastels and a shade of green not before seen in the Trans-Pecos region. A geologist and avid rock enthusiast in the area since the early 1960’s is quoted as never having seen these colors and variety of material from a given area. The West Texas Trans-Pecos area is widely known for its significant agate localities such as Needle Peak, Woodward, Walker, and Stillwell Ranches. While each yields a specific variety for which it has been long recognized, none approaches this recent find in overall diversity.

The intent of this article is to examine several of the circumstances or conditions believed to have contributed to this diverse material. Of prime importance is a basic understanding of the geologic history of the area and, in particular, the volcanic (igneous) activity described in the literature. Information noted described a period of activity, post-Cretaceous, which lasted over 15 to 20 million years, beginning some 40 million years ago. The subject area lies short distance from the Late Paleozoic Ouachita-Marathon fold and thrust belt and the Marathon uplift. The surface rock exposed initial surface flows was post emergent, thick and highly eroded Cretaceous limestone. Given that the surface onto which the earlier lavas flowed was eroded and irregular, lava “layers” would not be consistent, but very irregular. Lateral coverage would be broken as flow will follow least resistant course. The description of identification of drill cuttings from several water wells has added significantly to understanding the petrology of the area. Cuttings from wells in close proximity show inconsistent rock noted at equivalent depths.

Most of the lava flows were of basaltic material but also include trachites and rhyolite. These are interbedded with tuffs, tuff breccias, mud flows and ash beds. Based on well information, a good 500 feet of this material lies below points of lower elevation. Adding to this the additional volcanics between here and the average maximum elevation, there could be in excess of 800 feet of these materials in some areas. These rocks are primarily considered mafic in nature; that is, having a lower silica content and are usually rich in iron-magnesium minerals, which contribute to some coloring observed in these agates. Also, in the eastern part of the subject area, these occurs a significant amount of intrusive rock, including the principal source of the intrusive (plug), with associated sills and dikes. While no information was found specifically describing the petrolific nature of this material, a type of felsic rock similar to granite would be expected. Crystals development in this rock would be larger than in the surface flows due to slower cooling of the magma.  The process of magma intrusion is destructive to rocks penetrated, and frequently is associated with faulting. The intrusive sills would be parallel in nature with the bedding of previous rock. Evidence of this occurrence is indicated in cuttings from a well drilled in the area where drill cut a limestone layer, then entered again into volcanic material.

In order to arrive at factors creating the variety of agates found here, we need to look further at the available source rocks. Rock texture and structure determines whether there is available porosity and/or void spaces for the deposition of agate forming minerals. Voids caused by gas bubbles in the lava provide one of the more important sites for agate development (amygdules, vesicles). Not all of the lava will have had sufficient entrained gas to produce voids of any size. Faults and stress fracturing leave open space for the deposition of agate. There is ample reason to believe such voids exist. Therefore, development as both nodular and fortification agates would be expected. Some of the volcanic rock can be expected to be too dense to permit development of any form. Much of the intrusive lava would fall into this category, and development of the fortification variety would be expected.

The types and availability of the common trace minerals which provide for the broad range of agate color is to be considered. Take into account that each of the many flow incidents would likely consist of at least minor changes in mineral content. Add to that the multiple layers of ash, mud and tuff materials, the availability mounts. Also, there exists both felsic and mafic volcanic rock types. There are undoubtedly many of the more common trace minerals available. Primary among those are the oxides of iron, manganese and magnesium. Others, less common, include rutile, actinolite, epidote, goethite, chlorite, hematite, and the list goes on. Silica occur in abundance to provide for silica gel. Solutions of the necessary minerals, given the time element, would be readily available. Also, minerals contained in the fine grained ash, mud and tuff would be more readily soluble. Rain water coming from surface seepage, and subsurface waters would be available to complete the necessary components.

Explanation regarding physical/chemical processes which combine to form the various colors/types of agate found worldwide still remain somewhat controversial. A treatise studying and expounding upon banded agates is contained within: “Banded Agates: Origins and Inclusions” by Roger K. Papian and Andrejz Zarins (a University of Nebraska publication, June 1994). Also helpful is an article published in the March 2016 Rock and Gem magazine written by Pat McMahan. Other useful materials used here include “Igneous Geology of Trans Pecos Texas” published by the Texas Bureau of Economic Geology, 1986. Also, the United States Dept. of Interior publication #187, “Geology of the Marathon Region, Texas,” 1937.