SOUTH PARK: MAMMILLARY CHALCEDONY

A BEAUTIFUL AND QUITE LARGE (PENNY FOR SCALE) PIECE OF MAMMILLARY CHALCEDONY.

On a sunny morning last week I woke up with a thought about mowing the lawn, not an activity I really cherish.  But then some words of Walt Whitman jumped into my mind: I believe a leaf of grass is no less than the journey-work of the stars.  So, I reasoned that no person would want to destroy the work of stars and therefore a field excursion/road trip was in order!  Where to?  How about South Park?  Grabbing a large black coffee and a friend, off we went  for a day of mineral hunting with an another friend acting as a “private guide”.  There are a couple of great “things” about South Park: 1) good morning coffee, second cup for me,  and after-hunting ice cream may be found at the Bayou Salado store in Hartsel; and 2) there are some really interesting microcrystalline quartz varieties available, including petrified wood and colored chalcedony.  In addition to the coffee/ice cream, Dave and Lark at the store might give you a hint about collecting, or sell you some of their minerals or handmade jewelry.

South Park is both a topographic and structural basin, and along with North Park and Middle Park, owe their existence to the major mountain building event in the area, the Laramide Orogeny (late Cretaceous to Eocene).  Generally termed Intermontane Basins, North, Middle and South Parks are large synclinal basins complimenting the large anticlinal mountain ranges surrounding them.  

The eastern boundary of South Park is the Front Range (and its numerous subdivisions) and that demarcation is generally a large thrust fault (Elkhorn Fault).  In some localities along the eastern edge of the basin the Precambrian rocks of the mountains have been thrust about seven miles over the basin rocks (McGookey, 2003).  The western boundary of the basin is the Mosquito Range with a variety of Paleozoic rocks dipping under the basin-fill rocks.  Near the south end of the Mosquito Range are two peaks that seem quite prominent and very visible—Buffalo Peaks.  The rocks composing these peaks are volcanic in nature, including the Buffalo Peaks Andesite and various ignimbrites (hot churning gases and debris flowing by density from an eruptive center), and were deposited in a paleovalley during the Eocene-Oligocene.  Today, because of erosion, these old valleys are now high mountains and are an example of topographic inversion.  The north boundary of South Park includes several intrusive stocks of Laramide age.  The south boundary is perhaps the most interesting because of the large volcanic centers, including the Thirtynine Mile and Guffy volcanics (part of the Central Colorado Volcanic Field: CCVF).  Eruptions from these centers blocked the south outlet of the Basin and created a large lake and finally forced an eastward flowing outlet that was superimposed across the Front Range (McGookey, 2003).

PARK COUNTY, COLORADO.

At Hartsel, CO 9 and Park County 53 head south from Hartsel toward the small community of Guffy (CO 9) and the Thirtynine Mile Volcanic Area, a small remnant of the much larger CCVF.  Ash and other eruptive rocks from the CCVF cover an area of approximately 8500 sq. mi. including most of the “Sawatch Range, southern Front Range, Wet Mountains, northern Sangre de Cristo Range, and the areas between. Outflow aprons extended onto the High Plains to the east, merged with the San Juan volcanic field to the southwest, and overlapped the Colorado Mineral Belt on the north and west” (McIntosh and Chapin, 2004).  The major volcanism came from at least 10 calderas or eruptive centers with dates over a 10 million year span in the late Eocene into the Oligocene (38-29 Ma); however, volcanic activity continued into the Miocene (Wallace and others, 1999).  Post depositional faulting, dissection and erosion have produced the current landscape.

It was in this area of weathered volcanics that I was hunting for petrified wood, jasper, agate and chalcedony---and was quite successful.  Several localities produced good specimens and some of the wood was opalized in various colors.  But the most fantastic specimen I observed was a quite large piece of mammillary chalcedony, a beautiful and fantastic specimen.  The ultimate source of this specimen was most likely a vug in the volcanic rocks but weathering had left the piece exposed in a short-grass field.  But alas, it was not mine to take home; however, I do have the photos for remembrance!  

Coming back home across Wilkerson Pass a sort of peace descended upon me as I was again thinking of Whitman: Keep your face always toward the sunshine - and shadows will fall behind you.

REFERENCES CITED

McGookey. D. P., 2003, Geologic Wonders of South Park, Colorado with Road Logs: unknown binding. 

McIntosh, W. C. and C. E. Chapin, 2004, Geochronology of the Central Colorado Volcanic Field: New Mexico Bureau of Geology & Mineral Resources, Bulletin 160. 

Wallace, C. A., J. A. Cappa and A.D. Lawson, 1999, Geologic Map of the Gribbles Park Quadrangle, Park and Fremont Counties, Colorado:  Colorado Geological Survey Open-File Report 99-3 (with map).