University of California Press Geology of the Pinnacles National Monument

GAVILAN LIMESTONE

The Gavilan limestone of the Sur series is everywhere found in contact with granite and is so metamorphosed thereby that its original features are largely obliterated. The lime deposits were originally interbedded with mud and sands, now represented by mica schists and quartzites.

One sample of marble from an exposure two miles southwest of Chalone Peaks is megascopically gray and is speckled with small dark spots that average between 1 and 2 mm. in diameter. Under the microscope about 90 per cent of the marble is seen to be composed of polysynthetically twinned calcite; the most abundant of the other constituents is olivine, which occurs in rounded grains averaging 1 mm. in diameter, much of it altered to serpentine and commonly associated with magnetite grains. Pyrite, grossularite, and graphite are also abundant in grains that average 1/4 mm. or less.

A light gray marble along Shirttail Gulch contains numerous rounded grains of pale green augite averaging 1 or 2 mm. in diameter. Some marbles exhibit green banding, because of the presence of abundant serpentine.

Although the writer found no fossils in these rocks, crinoid stems have been discovered in the Gavilan limestone of Fremont's Peak (25 miles to the northwest).¹¹ Reed¹² considers that the age of this formation is probably either Carboniferous or Permo-Carboniferous.

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¹¹Smith, J. P., Geologic formations of California, Calif. State Mining Bur., Bull. 72, p. 26, 1916.

¹²Reed, R. D., Geology of California, p. 64, 1933.

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Exposures in this southern part of the Gavilan Range consist of thin "roof pendants" or remnants occurring sporadically over the denuded batholith. Outcrops are seldom more than a few hundred feet long, and thicknesses of more than 20 feet are unusual.

The crystalline limestone is slightly more resistant to weathering than the underlying quartz diorite and granite, so that it tends to form small rounded hills. In many places it gives rise to caliche at the surface, although elsewhere caliche may be caused by weathering of lime feldspars in the quartz diorite.

SANTA LUCIA "GRANITE"

In the southern portion of the Gavilan Range the "granite" has been more completely denuded than at the northern end, so that only isolated areas of metamorphosed sediments occur in the Pinnacles region. In most places, the schists and quartzites of this southern area occur as intercalations within the Gavilan limestone or in close proximity to these beds and have therefore been mapped together with this formation.

The term "Santa Lucia" is here applied to the granitic rocks of the Gavilan Range, since they are considered to be generally related to the granites of the Santa Lucia Range, but these rocks are somewhat different texturally from the facies at Carmelo Bay.¹³

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¹³Lawson, A. C., Univ. Calif. Bull. Dept. Geol., vol. 1, pp. 1-18, 1893.

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The plutonic rocks fall into two main divisions and a number of minor types. There is a considerable area of true granite, and quartz diorite is exposed over a somewhat larger area.

In the area to the east of the Pinnacles volcanic belt, true granite is exposed. Locally, this is a medium grained, light gray, sodic type in which albite is the chief feldspar. In this soda granite, quartz is abundant, and in many specimens approaches 50 per cent of the rock, with muscovite and biotite as the characterizing accessories. Muscovite is more abundant than biotite but the two account for less than 10 per cent of the rock. Minor accessories include hematite, apatite, and a very small amount of magnetite.

More abundant than the gray, sodic granite, is a medium to coarse grained granite with light pink orthoclase, which is usually fresh and glassy. One type contains large phenocrysts of pink microcline up to 4 cm. in diameter, together with smaller grains of anorthoclase, albite, orthoclase, muscovite, biotite, etc.

Most of the granites are medium to coarse grained; the chief feldspar is orthoclase with minor amounts of oligoclase, albite, microcline, and anorthoclase; the chief accessory minerals are biotite and muscovite and, rarely, hornblende. Magnetite, apatite, and hematite are the only common minor accessories. Aplites and pegmatites are not of common occurrence in the area exposing true granites.

All the xenoliths of plutonic rock within the central mass of lavas are of granite, rather than quartz diorite. Granite was also abundantly expelled with the fragmental ejecta. The rhyolite was unable to alter the orthoclase to sanidine, and in no crystal was the observed optic angle less than 70 degrees. Moreover, much of the biotite in these granitic xenoliths is practically uniaxial or has an optic angle of less than 5 degrees, so that heat was either insufficient or applied for too short a time to bring about any appreciable change.

On the west side of the Pinnacles volcanic area, and in sharp contrast to the granite on the east, all the plutonics examined were either of quartz diorite or granodiorite.¹⁴ They are typically medium to coarse grained and gray in color although abundant biotite may produce a dark shade. Locally, the quartz diorite contains as much as 50 per cent of shiny black biotite.

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¹⁴Lindgren, W., Am. Jour. Sci., vol. 9, p. 279, 1900.

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Quartz diorite and granodiorite grade into each other. The feldspars in these rocks include a little orthoclase, but andesine-labradorite is the most abundant and more nearly euhedral variety. Quartz usually constitutes less than 25 per cent of the rock. Associated minerals include biotite, greenish brown hornblende, light gray augite, and sometimes muscovite. Apatite, magnetite, pyrite, titanite, light colored garnets, and colorless zircons are common minor accessories. Titanite may amount to 5 per cent of the rock and magnetite to as much as 2 per cent. Apatite commonly occurs as needles in the quartz and feldspar, and was apparently the first mineral to form.

Along Miners Gulch (southwest part of map) and for some distance to the southwest, the plutonic rocks are gneissoid and locally there are mica schists. Faulting may account for part of this structure although the zone of gneissoid granite is rounded, rather than elongated along the adjacent fault. Probably the gneissoid structure indicates a border facies of the batholith. Injection gneisses have been noted locally and pegmatite dikes of considerable size and persistence are prevalent.

A typical specimen from Miners Gulch, with a fine grained gneissoid structure, contains about 50 per cent quartz, many crystals exhibiting the effects of strain; albite is the most abundant feldspar, and light golden brown biotite in shreds and irregular grains amounts to 15 per cent of the rock. Pale green augite (Z^c=40°) is sparingly present, together with titanite, pyrite, apatite, and magnetite.

Southwest of Miners Gulch and at the east central edge of section 32, a single exposure carrying both well-developed talc-actinolite schist and talc schist was found.

In describing the Santa Lucia granite of the Point Sur Quadrangle, Trask notes that

it apparently grades into the main quartz diorite mass of the region, and hence is probably a differentiate of the quartz diorite. . . .¹⁵

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¹⁵Trask, op. cit., p. 134, 1926.

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Fairbanks, on the same topic, says that

In addition to the coarse, fresh-looking granite and gneiss of the Santa Lucia Range, there are large bodies of granitic rocks with a chloritic constituent in the place of the mien, and which are probably much older.¹⁶

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¹⁶Fairbanks, op. cit., p. 519.

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Although exact relations between the granite and quartz diorite could not be determined in the Pinnacles region, the evidence points toward a rather definite separation of the two and possibly a difference in age. Generally, the quartz diorite is more weathered and in many places is greenish at the surface because of the alteration of biotite to chlorite and saussuritization of lime feldspars. However, this cannot be definitely construed as indicating a greater age. Whether the granite had intruded the quartz diorite or vice versa was not evident from field observation. A northwest-southeast line through the southern part of section 36, near the mouth of Chalone Creek, marks the southern limit of the granite. The contact of the granite and quartz diorite, to the northwest, is obscured by the rhyolitic extrusives.

Even if the granite and the quartz diorite were intruded at different times, it is reasonable to assume that the age difference between the two is not great. For both, the period of intrusion cannot be stated more definitely than that it is pre-Franciscan and younger than the Sur series.