Page 157 of 216
Gueydan Formation
155
The limestone pebbles which are included in the serpentine show many irregular emibayments along their borders and are cut by minute sinuous, vein-like masses of serpentine. These features strongly suggest that the serpentine was intruded into a limestone conglomerate. It is posssible that the irregular reentrants of serpentine in the limestone were produced by plastic flow of the serpentine under pressure generated by the hydration and consequent expansion in vol ume of a basalt or basalt tuff from which the serpentine may have formed. However, no shearing can be noted in the limestone, although such shear cracks may have been obliterated by being later filled with fibrous calcite stringers which cut both the limestone and the serpentine. If this is really an intrusive serpentine, instead of a hydrated basic tuff, the intrusive body is evidently a sill, a thin laccolith, or an inclined dike because Gueydan, Frio, and Fayette tuffs and sediments are found beneath the serpentine.
The nearest known intrusive bodies are the nepheline and melilite basalt and phonolite volcanic necks and lacco liths in Uvalde County, more than 100 miles northwest of the Hawley well. The nearest known mass of serpentine rock is that encountered between 1,150 and 1,450 feet in the Lytton Springs oil field, in the northern corner of Cald well County, about 110 miles north of the Hawley well. The serpentine rock from Lytton Springs has been examined by the writer and is entirely different in texture from that in Live Oak County. No pseudomorphs after olivine crys tals are seen in the Live Oak County serpentine but such pseudormorphs are very abundant in the Caldwell County serpentine. The Caldwell serpentine is an altered basaltic volcanic breccia or agglomerate composed of angular frag ments of several types of hydrated basalt. The Live Oak serpentine shows no suggestion of agglomeratic texture.
Origin of the Gueydan
From the foregoing descriptions and discussions it is evident that the Gueydan formation is a series of land-laid pyroclastics interbedded with fluviatile sediments. Many
