WALKING THE CREEK: COLORADO SPRINGS

JASPER COLLECTED FROM FOUNTAIN CREEK

The fall weather in Colorado Springs has been of the fantastic variety during the last several weeks.  In fact, it was a great time to take hikes, visit the local rocks, and take a peek at whatever would pop up. 


Colorado Springs lies in the drainage basin of Fountain Creek. The stream heads near Woodland Park flowing southeast to Colorado Springs and picks up the major tributary, Monument Creek.  In fact, the latter is probably the master stream but it lost the naming contest.  Upper Fountain Creek, and the tributaries, cut through the Precambrian Pikes Peak "granite" and a variety of Paleozoic rocks exposed on the way east.  Monument Creek, heading near Palmer Lake and the Monument Divide, flows south along the east flank of the Front Range through Pleistocene, Tertiary and Cretaceous rocks and sediments.  After joining together the combined streams flow south to the Arkansas River at Pueblo picking up whatever the Cretaceous Pierre Shale has to offer.


CSMS members have "walked the creek" for many years and have collected a great variety of rocks, minerals and fossils.  In my short walk this week it was easy to observe: pieces of Pikes Peak "granite" of all sizes from large cobbles down to the quartz and feldspar components, Precambrian Idaho Springs metamorphic rocks, collectable jasper of several shades of orange, yellow, brown, broken concretions and cone-in-cone from the Pierre Shale, petrified wood from the late Cretaceous and/or Tertiary, a variety of Paleozoic limestones (some boulder size), and other rocks.  Some of the CSMS rockhounds have collected amazonite, crystal clear smoky and clear quartz crystals, a variety of Pierre cephalopods, and other interesting "things"!   
 
And then there are the recent artifacts and "junk".  The most interesting were the many pieces of round and colored glass and porcelain.  Most "junk" was the result of many lazy people, and our country's love of all things "plastic".

FOSSILIZED WOOD FROM FOUNTAIN CREEK

DIKES AT WILLOW CREEK

UNNAMED DIKE INTRUDING COALMONT FORMATION NEAR WILLOW CREEK PASS

Colorado is blessed to have a wide variety of igneous rocks exposed in the central and western part of the state.  These are rocks that originated deep in the earth as liquid magma and then cooled and solidified either below the surface of the earth (intrusive) or on the surface of the earth (extrusive).  The latter are commonly known as volcanic rocks and since they cool very rapidly the individual mineral grains are tiny and magnification must be used to observe the minerals.  In fact, obsidian cools so rapidly that individual crystals are absent from the structure.  These extrusive rocks are “extruded” by explosive volcanoes, or as flows.

In contrast, intrusive igneous rocks “intrude” into preexisting rocks and cool very slowly.  Granite is a common intrusive igneous rock and one can easily see the individual grains of feldspar, quartz, and iron minerals in samples of the local Pikes Peak Granite.  Pegmatites develop near the margins of the magma chamber and cool very, very slowly.  Therefore, crystals can grow quite large; hence, the reported four foot crystals of smoky quartz near Devil’s Head (Pikes Peak pegmatites), and spodumene “logs” in South Dakota (Black Hills pegmatites) approaching 50 feet in length.

Geologists apply a wide variety of names to bodies of intrusive igneous rocks (plutons) including concordant (intrusive rocks are parallel to bedding planes) and discordant (intrusive rocks cut across preexisting rocks).  Further terms include (http://en.wikipedia.org/wiki/Intrusion):

• batholith: large irregular discordant intrusions.
• stock: smaller irregular discordant intrusions.
• dike: a relatively narrow tabular discordant body, often with near-vertical attitude.
• sill: a relatively thin tabular concordant body intruded along bedding planes, often near-horizontal when emplaced, but may be intruded into tilted beds or the entire package may be tilted by later deformation.
• pipe or volcanic neck: circular or tube shaped nearly vertical body which may have been a feeder vent for a volcano.
• laccolith: concordant body with essentially flat base and dome shaped upper surface, usually has a feeder pipe below.
• lopolith: concordant body with a relatively flat to sagging top and a shallow convex base (spoon-shaped), may have a feeder dike or pipe below.
• phacolith: concordant lens-shaped pluton that typically occupies the crest of an anticline or the trough of a syncline.

To me, the most scenic plutons are the igneous dikes intruded into preexisting sedimentary rocks.  Dikes intruded into igneous or metamorphic rocks are usually of a similar hardness as the host rock and usually the weathering is similar.  However, dikes intruded into sedimentary rocks are almost always harder than the host rock and therefore tend to erode at a slower rate.  As a result, the dikes become topographic features and are readily noticed.  The most famous dikes in Colorado are those radial dikes associated with the Spanish Peaks in southern Colorado near La Veta.  In fact, these may be the most famous dike swarm in the United States.

However, I prefer my dikes to be infamous, unnamed, startling, surprising, and just something that will “knock your socks off” when driving a back- country road.   Such dikes are found north of Granby on lightly traveled Colorado Highway 125 near Willow Creek Pass.  

Colorado 125 intersects with US 40 slightly west of Granby (three miles) and then generally follows Willow Creek north from the intersection until it reaches the Pass in approximately 21.5 miles.  Along the way the road cuts through several exposures of the late Cretaceous Pierre Shale and the overlying basin fill rocks (arkosic sandstone, conglomerate, shale and some coal) assigned to the Coalmont Formation (Paleocene –Eocene) (Tweto, 1979).  Geologically, the Pass is interesting as it is near the intersection of North Park and South Park, bounded on the east by the Never Summer Range and on the west by the Rabbit Ears volcanics.  At 9621 feet it is one of the lowest places along the Continental Divide in Colorado.  After summiting the Pass, the road continues on for about 52 miles through Walden and finally to the Colorado-Wyoming border.  The scenery is spectacular for the entire trip.

There are several dikes exposed near Willow Creek Pass at approximately Mile Markers  18.2, 20.2, 20.7, 21.4, 21.8, 21.9  (Donnell, 1960 and see geologic maps of Tweto, 1979 and Toth and Soullier, 2000).  The rocks composing the dikes appear to be dacitic (high in plagioclase feldspar and iron and magnesium minerals) to rhyolitic (high silica and low iron and magnesium); all are porphyritic in texture (Toth and Soullier, 2000).  Porphyritic rocks have large grained crystals (phenocrysts) set in a fine- grained matrix and probably indicate the cooling of the magma in two different stages.  Immediately to the east of the dikes is Radial Mountain at 11,240 feet.  Although I could not locate information as to the naming of this peak, it appears that the dikes radiate out from the summit (for an interesting view see Google Maps at http://maps.google.com/ and zoom in on Willow Creek Pass on both the Terrain and Satellite programs), an outcrop of rocks similar to the dikes (Tweto, 1979).  Toth and Soullier (2000) believe the dikes are probably Oligocene in age, perhaps 24-28.8 Ma.  They may be related to crustal extension associated with the Rio Grande Rift to the south. 

One of the most interesting aspects of the Willow Creek dikes is

that these quite hard igneous rocks are intruded into the easily

erodible Coalmont Formation.  Therefore, the dikes stand out in

high relief and cause the reaction:  “Whoa!  Stop the car.  What is 

the world is that”.  Colorado 125 is a beautiful back-country road 

with some “up-close” geology available for all to observe. So think 

about doing what Steve   Miller sang: Goin' to the country and 

leavin' right away.  No time to  talk I got to make a getaway.

REFERENCES CITED

Donnell, J. R. Geological Road Logs of Colorado: Denver, Rocky

Mountain Association of Geologists, 1960.

Toth, M.I., and S. J. Soullier. Geology in Bankey, V., Soullier, S. J., 

and Toth, M.  I., eds.  Mineral Resource Potential and Geology of 

the Routt National Forest  and Middle Park Ranger District of the 

Arapaho National Forest, Colorado:  U. S. Geological Survey 

Professional Paper 1610, 2000.

Tweto, Ogden. Geologic Map of Colorado. U.S. Geological Survey
State Map Series, scale 1:500,000.

TRACKS/TRAILS WISCONSIN LIMESTONE

FRONT STEP AT GROUNDED SPECIALTY COFFEE.  NOTE BURROWS AND STROMATOLITES.  DIME FOR SCALE

For eight years I resided in La Crosse, Wisconsin, a picturesque city located on the Mississippi River.  For most of those years I had my morning coffee with the "7:00 am Group” at Grounded Specialty Coffee.  These fellow Wisconsinians were an eclectic group of businessmen/women who taught me much, not the least of which is that academicians need to listen closely to the townies.  As a bonus, it is not every day that a coffee imbiber gets to discuss Keynesian Economics with the chief barista (a doctorate in economics), the Zen of motorcycle maintenance with Biker Bob, local judicial matters with Mike the Barrister, La Crosse history with Carl the Philosopher, or Catholic shrines with Andy the Accountant!   At any rate, for several years I made the morning trek, climbing a single step into the shop to sample the espresso.

During my recent trip to Grounded in September, I made a startling discovery—the single step to the shop was not composed of man-made concrete but of local building stone.  In closer examination I also observed that the step was badly bioturbated and covered with trails/burrows left behind by soft bodied (probably) animals.  There are numerous vertical and horizontal burrows preserved, most likely the result of “worms”.  The normal differential weathering of the carbonate rock preserved these animal traces in raised relief.  In addition, I noticed larger structures with a dark center surrounded by concentric layers.  These interesting features are stromatolites, layered structures formed in very shallow water by cynobacteria (blue-green algae).  It seems as though these photosynthetic bacteria trapped rock and mineral grains in their mucus and cemented all together.  I was unable to observe tracks of trilobites or body fossils.

I believe the rock in question is a specimen from the early Ordovician Prairie du Chien limestone.  The Prairie du Chien is the local “caprock” and may be observed “holding up” the bluffs on both sides of the river.  It has been quarried extensively for building stone and riprap and a large quarry is easily observed at the summit of Granddads Bluff in east La Crosse.  The Prairie du Chien was deposited in a very shallow (maybe 20-30 feet) and warm sea as a carbonate mud that later hardened into a limestone (CaCO₃).  Much of the unit has been altered (post deposition) into dolomite (MgCaCO₃).

So, if you happened to travel through La Crosse, stop in at Grounded (308 Main St.), visit with Todd and the boys, and check out that great step.

BLUFF IN EAST LA CROSSE. PRAIRIE DU CHIEN CAP.  NOTE QUARRY ON RIGHT.

RHODOCHROSITE FROM ARGENTINA

Fig. 1. Massive rhodochrosite stalactites exposed in the Capillitas Mine, Argentina.  Photo courtesy of J. A. Saadi and The Giant Crystal Project.

 

The carbonates are a common and important group of minerals found in rocks of the earth’s crust.  All carbonates are distinguished by having a  CO₃ ion in their chemical composition.  Many carbonates are quite colorful such as the shades of calcite (calcium carbonate) and the blues of azurite (copper carbonate).  Most mineral collections contain numerous specimens of carbonates but three related minerals are of special interest: iron carbonate (siderite), calcium carbonate (calcite), and manganese carbonate (rhodochrosite).  The reason behind this interest is their solid solution relationship.  For example, in the popular collectable mineral rhodochrosite (MnCO₃), calcium, as well as iron, may substitute/replace the manganese; therefore, the mineral may exhibit many different shades of pink and red and the exact chemical formula varies with the differing amounts of manganese, iron, and calcium.  It is my understanding that “pure” rhodochrosite is rather rare.

The most “famous” rhodochrosite” crystals are the specimens collected from the Sweet Home Mine near Alma, Colorado.  These rose-red colored crystals are prized by collectors the world over and the Denver Museum of Nature and Science has a fantastic display.  However, also of interest to many rockhounds are the “sliced and polished stalactites” from the Capillitas Mine in Argentina. These non-crystal specimens are very recognizable and occur as the principle gangue mineral in this lead-zinc sulfide mine (Fig. 1).  The minerals are the result of hydrothermal activity in the late Tertiary and may be the largest mass of rhodochrosite ever discovered (The Giant Crystal Project, 2011).  I treasure these slices for both their color and their uniqueness (Fig. 2). 
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Fig. 2.  Rhodochrosite (stalactites) from Capillitas Mine in Argentina. Specimens ~4 X 4 cm.