Thursday, September 5, 2013


I travel to the Black Hills of South Dakota and Wyoming on a semi-regular basis to visit friends and relatives and to try and lasso an elusive trout or two.  The Hills are also a wonderful place to collect mineral specimens from some of the numerous pegmatites.  Campgrounds are abundant and the geology is well-exposed.

Generally speaking the Hills are cored by Precambrian rocks, both of Archean and Proterozoic ages, and their exposures create some of the best known attractions, i.e. the Needles Highway and Mt. Rushmore.  It is interesting to note that these same Precambrian rocks are perhaps 15,000 feet below the surface in the Williston Basin to the immediate north of the Hills.  A very long (time-wise) unconformity separates the Precambrian rocks from the overlying Paleozoic rocks---usually the Deadwood Formation.  At some meeting I attended (my mind fails at attempting to recall which meeting) the speaker threw out a time gap of about one billion years plus.  The Deadwood represents the transgressive phase, shoreline and shallow marine, of the early Paleozoic seaway.  As this seaway moved from west to east (today’s direction) the shoreline sandstone ranges in age from Early Cambrian (southern California), late Early Cambrian [Pioche Formation, western Utah], early Middle Cambrian [Tintic Quartzite, central Utah] (Hintze and Kowallis, 2009), early Late Cambrian [Sawatch Formation, Colorado] (Myrow and others, 2003) to latest Cambrian-Early Ordovician Deadwood Formation (Sokoloski, 2005).  All of these sandstones are termed Cratonic Quartz Arenites and are sheet-like in nature.  These rocks represent a classic example of marine waters slowly encroaching onto the Precambrian rocks of the Craton (the central stable part of the continent). 

Diagram of the Black Hills uplift by A. N. Strahler but taken from Trimble, 1980.
The remainder of the Paleozoic in the Black Hills is represented by sandstones, shales and carbonates related to numerous transgressions and regressions of marine waters; unconformities are numerous.  Perhaps the best known Paleozoic rocks are the Mississippian Pahasapa Limestone (Madison Formation), home of Wind Cave, and the Pennsylvanian Minnelusa Formation (home of Teepee Canyon Agates and ultimately Fairburn Agates)---see blog posting8/18/12).

The Triassic-Jurassic starts out with the redbeds of the Spearfish Formation (forming the racetrack around the Hills), transforms to marine carbonates and shales and ends with deposition of the non-marine, dinosaur-bearing Morrison Formation.

Marine waters again invaded in the Cretaceous period; the rocks are represented by sandstones, shales and carbonates deposited during four marine transgressive-regressive phases. 

Cenozoic non-marine rocks may have, at one time, been present in the Black Hills.  However, physical outcrops do not remain.  There seems not to be evidence of Pleistocene mountain glaciation in the Hills.
The doubly plunging anticline representing the structural feature of the Hills is Laramide in age (the Rocky Mountains uplift orogeny), probably beginning about 62 Ma (Cretaceous) and continuing into the Eocene (Redden and Lisenbee, 1996).

A domed Spearfish Peak, elevation 5796 feet, rising from Northern Black Hills south of Spearfish, South Dakota.

Tertiary phonolitic rocks moving down slope (gravity) on the southeast flank of Spearfish Peak.  The Junior Geologist for scale.
During my last few trips exploring for minerals I have been fascinated with tracking down a few of the Tertiary intrusives (Eocene) that dot the landscape both within the Hills (i. e. Crow Peak---see blog posting 8/14/12) and on the Plains (i.e. Bear Butte---see blog posting 6/10/12).  The best known of these intrusive structures is probably Devil’s Tower situated in Wyoming west of the Hills about 35 miles.  Most of the intrusives are laccoliths although a few are sills.

Geologic Map of the Black Hills near Spearfish, South Dakota.  Spearfish Peak, mapped in red as Tp (Tertiary Phonolitic Intrusive Rocks), dominates the skyline south of Spearfish “city”.  Map from DeWitt and others 1989.
Saw-cut surface of phonolite with massive nephaline and black sprays of aegirine-augite.  Note a feldspar lathe towards upper right corner.  The "light-colored" specks are probably nosean.
During my last trip in summer 2013 I had the Junior Geologist haul me up some back country roads to a locality termed Spearfish Peak.  This mountain dominants the landscape south of the town of Spearfish (northern Hills) yet is somewhat difficult to locate on the USFS roads.  However, it was worth the drive as the exposed rocks of the Mountain are sort of “different”, at least to me!  Roberts and Rapp (1965) described the intrusive rocks at Spearfish  Peak as a phonolite with aegirine-augite, nosean, sodalite, and nephaline.  Pretty interesting!  DeWitt and others (1989) mapped Spearfish Peak as “phonolitic intrusive rocks of Eocene and Paleocene age” and part of a group of sills and laccoliths occurring from west of Lead to near Galena.  The mineralogy seems complicated with numerous feldspathoid minerals, aegirine-augite, biotite and sphene along with feldspars and others.  They also noted that the phonolites were characterized by anomalously high concentrations of barium and strontium.  Unfortunately I have neither a polarizing microscope nor an XRD nor an XRF to confirm this analysis.

Phonolite collected on Spearfish Peak.  The dark mineral sprays are aegirine-augite.  Width of rock ~11 cm.
 However, there are quite interesting macro-crystals in the phonolite that are visible to the eye and especially impressive as seen with a binocular microscope.  Very large crystals of aegirine [NaFe3+SiO2O6] or perhaps aegirine-augite are scattered on the surface almost as radiating sheathes. I really never understood much about aegirine until my friend Pete at the USGS gave me a crash course on some of the pyroxenes.  That information is present in a blog posting on 3/17/13.

Photomicrograph of Spearfish Peak phonolite showing Aegirine (A), Nephaline (N), and possibly Nosean (?). Width ~1 cm.

Photomicrograph showing possible Nosean (?).  Width of photo ~1 cm.
Using a binocular scope it appears that I can also identify nephaline [(NaK)AlSiO4] and nosean [sodalite group Na8(Al6Si6O24)(SO4)-H2O].  I could not locate the described sodalite. 

Whatever the case, this field trip to Spearfish Peak was exciting and I was able to collect a somewhat uncommon rock—phonolite--- with some uncommon minerals exposed in an interesting laccolith.     

DeWitt, E., J.A. Redden, D. Buscher, and A.B. Wilson, 1989, Geologic map of the Black Hills area, South Dakota and Wyoming: United States Geological Survey Miscellaneous Investigations Series Map I-1910.

Hintz, L.F. and B.J. Kowallis, 2009, Geologic history of Utah: Brigham Young University geology Studies Special Publication 9
Myrow, P.M., J.F. Taylor, J.F. Miller, R.L. Ethington, R.L. Ripperdan, and J. Allen, 2003, Fallen arches: Dispelling Myths concerning Cambrian and Ordovician paleogeography of the Rocky Mountain Region: Geological Society of America Bulletin. 

Redden, J.A., and A.L. Lisenbee, 1996, Geologic Setting, Black Hills, South Dakota, in C.J. Patterson and J.G. Kirchner, eds., Guidebook to the Geology of the Black Hills: South Dakota School of Mines and Technology Bulletin 19.
Roberts, W.L. and G. Rapp Jr., 1965, Mineralogy of the Black Hills: South Dakota Schools of Mines and Technology Bulletin 18.

Sokoloski, W. P., 2005, Sedimentology and ichnology of Late Cambrian to Early Ordovician sandstone in the Deadwood Formation, Northern Black hills, South Dakota, and Southeastern Bear Lodge Mountains, Wyoming: MS Thesis, University of Toledo.

Trimble, D.E., 1980, The geologic story of the Great Plains: United States Geological Survey Bulletin 1493.