Traveling from Burnet west on Rte. 29, Turn North on RM 2341.
+3.4 miles, pass Burnet Co. 113
+3.5 miles, cross Clear Creek
(Between these two on the left is a roadcut of reddish quartzite within the Packsaddle Formation)
+3.8 miles, low roadcut on left is deformed granite of the Midway Sill.  The exact age of the Midway Sill is not known, although it is believed to be >1.093 Ga.
+4.2 miles, roadcut and outcrops on the right are also deformed granite of the Midway Sill
+4.4 miles, pass Burnet County 114 (Graphite Mine Road)
+4.7 miles, Stop X-1 Roadcut on west side is in the Midway Sill
+4.8 miles, Stop X-2 Roadcut primarily on the west side is in the contact zone of the Midway Sill.

The Midway Sill is located in the northeast corner of the Llano Uplift along the contact between the Packsaddle Schist (PS) and the Valley Spring Gneiss (VSG) just to the east of Lake Buchanan. The Midway Sill was originally named by Stenzel (1935) who described it as being part of the Town Mountain Granite (TMG) suite. Denny (1982) noted that the sill was ductily deformed and designated it the Midway Augen Gneiss.
The Midway Sill was a porphyritic microcline-quartz-biotite-plagioclase-hornblende granite that has been deformed into a foliated augen gneiss. Large pink tabular microcline megacrysts reach 3 cm. in length (Fiore, 1976) and impart the distinctive TMG appearance. Denny (1982) estimated the mode of the sill as:

Microcline 45%
Quartz 20%
Biotite 19%
Plagioclase 15%
Hornblende 1%

This mode plots in the granite field of the IUGS classification. The Midway Sill has more mafic minerals than a typical TMG. Accessory minerals seen in thin section are titanite, apatite, and spectacularly zoned, partially metamict zircons.

Foliation in the Midway Augen Gneiss strikes NW to WNW and dips moderately to the SW, subparallel to the foliation and layering seen in the country rock . Contacts with the country rock are generally parallel to the foliation in the granite (Fiore, 1976). An oblique mineral lineation plunges moderately to the southwest. Early dikes that intrude the country rock are folded, boudinaged , or mylonitized. Late dikes are undeformed, suggesting deformation was syn-plutonic.

In thin section, the microcline feldspar has a well-developed perthitic texture. Deformation has lead to development of core and mantle structure on many of the phenocrysts. Mantles are comprised both of recrystallized microcline and of myrmekite. Quartz grains commonly are very elongate, but with weak to moderate strain features preserved internally suggesting deformation at high temperatures. Preliminary kinematic analysis, based on asymmetric porphyroclast tails and weak S-C fabrics, is shearing with right lateral and thrust components (top to the northwest).

Denny (1982) chose to correlate the Midway Granite/Augen Gneiss with the Lost Creek Gneiss, a unit from the western portion of the Uplift (see front cover). The Lost Creek Gneiss is found along the PS-VSG contact and is much older than TMG. This correlation seems to have been based on the location of the Midway Sill at the VSG-PS contact and the presumption that TMG should not be deformed. However, the Midway Augen Gneiss bears little resemblance to the Lost Creek Gneiss. The Midway Sill looks like deformed TMG, and is very similar to foliated granites from the type TMG in the Wolf Mountain Intrusion. Augen gneisses of a much different appearance that may correlate with the Lost Creek Gneiss have been described from elsewhere within the Valley Spring Gneiss in this area (Fiore, 1976). Also, while the Midway Sill is deformed, it is not as pervasively deformed as the gneisses seen in the country rock. I think the Midway Sill is a synplutonically deformed example of Town Mountain Granite.

Stop X-1: Roadcut showing fresh and weathered exposures of the Midway Sill granite. Outcrop shows a well-developed foliation which strikes ESE to SE and dips predominantly moderately to the southwest . Note recrystallization of the feldspar porphyroclasts. Most of the outcrop is somewhat oblique to the X-Z plane of strain, but dextral asymmetries can be observed on some feldspars. In places the granite is cut by highly oblique shear zones with an antithetic sense of shear.

Deformed and undeformed granitic pegmatites are also present. Undeformed pegmatites have irregular margins but overall strike about S60W. At least one of these pegmatites contains garnet-quartz symplectites (look for dark purple-brown patches). The same texture is also found in S60W-trending pegmatites of the Wolf Mountain Intrusion from the central portion of the Llano Uplift. Another pegmatite in this outcrop has masses of magnetite around 1 cm in diameter.

Stop XB. This deeply weathered roadcut exposes rocks near the Midway Sill/Valley Spring contact. Valley Spring lithologies include a fine-grained biotite-quartz-feldspar gneiss with millimeter-scale banding and a strong foliation parallel to the banding. This rock weathers a dark gray that tends to obscure its felsic composition. The outcrop also includes more mafic lithologies. One of these rocks is a mafic gneiss, which in handsample appears to be made up of quartz-plagioclase-mafic minerals. One sample of this lithology (which may not be typical) was found to contain the assemblage quartz-plagioclase-clinopyroxene with minor microcline (a meta-tonalite?). The least common lithology here is a dark green amphibolite. Examination in thin section has shown this rock to be comprised almost completely of amphibole. Porphyritic granite sills are part of this outcrop as well. The granite sills are especially weathered, but appear similar to the deformed granite within the main part of the Midway Sill.

A number of sills of TMG have intruded this outcrop, along with aplites (now foliated) and numerous pegmatites (both deformed and undeformed). One spectacular pegmatite in the central portion of the outcrop is boudinaged as it cuts obliquely across foliation and layering . This boudinaged pegmatite also cuts a granite sill and is boudinaged in the granite as well. One puzzling aspect of this outcrop are a number of planar discontinuities with thrust fault-like morphologies. One of these discontinuities can be seen cutting off layering in an amphibolite body in the lower north-central portion of the roadcut. However, no clear offset sense has been found on any of these zones and they may relate to differing strain responses between different lithologies.

Denny, James H. (1982) Mesoscopic structural analysis of Precambrian rocks, Lake Buchanan area, northeast Llano Uplift, Llano and Burnet Counties, central Texas. M.S. thesis, Nacogdoches, TX, Stephen F. Austin State University, 132 p.

Fiore, Richard N. (1976) Geology and geomorphology of the Clear Creek drainage basin, western Burnet County, Texas. M.A. thesis, Austin, TX, University of Texas at Austin, 174 p.

Stenzel, H.B. (1935) Pre-Cambrian structural conditions in the Llano region. In The Geology of Texas, Vol. II, Structural and economic geology, University of Texas Bulletin, 3401, 74-79.