Tuesday, November 27, 2012

HIKING PULPIT ROCK


Pulpit Rock, a recognized Colorado Springs beacon, is composed of the late Cretaceous Dawson Formation.  The "andesitic facies" forms the lower vegetated slope and indicates the presence of eroded volcanoes to the immediate west of Colorado Springs in the Rampart Range.  The upper "cliff forming facies" represents deposition by energetic streams flowing off the Rampart Range depositing eroded granite.  Photo courtesy of Gilbert Davis.
One of the major landforms that visitors observe while traveling along I-25 through Colorado Springs, other than Pikes Peak, is the Austin Bluffs--Palmer Park Divide, a major landform trending north-south through the northern and central parts of the City.  The Divide, located just east of the Interstate, actually “hides” the eastern part of the city from the western locales.  The parks/open spaces all beckon visitors for hikes to observe their many geological features; however, perhaps the most intriguing trek is up the prominent landmark termed Pulpit Rock.  This Park may be accessed from a frontage road just east of the terminus of North Nevada Avenue as it merges with I-25.  A small parking lot is available and several trails lead to the summit.

Before beginning the hike visitors should look directly westward across Monument Creek and I-25 to gaze upon the nearby sandstones and shales of the late Cretaceous Laramie Formation that are well exposed in the Popes Bluff/Popes Valley area (see 4/6/11 blog).  These exposures of the Laramie Formation represent the final regression of the vast Western Interior Seaway (WIS) that flooded what is now Colorado during much of the Cretaceous Period, and whose sediments were deposited in a series of stream channels, coal swamps, and lagoons bordering the seaway.  The rising Rocky Mountains to the west were responsible for providing the sediments of the Laramie Formation and for driving the WIS from the interior of the continent. 

Stratigraphically above the Laramie Formation is the Dawson Formation, a unit spanning the Cretaceous-Tertiary (K-T) boundary (~65.5 my). The Dawson Formation may seem to represent continuous sedimentation from the Laramie Formation but actually there is a break in deposition and the missing rocks (unrepresented geologic time) are represented by an unconformity.  This boundary between the Laramie and the Dawson is not exposed at Pulpit Rock but is buried in the valley of Monument Creek.

At Pulpit Rock the Dawson consists of two major rock units, or facies, and both are synorogenic in nature, that is the deposition of the sediments was occurring at the same time (syn) as the mountain building (orogeny) to the west.  The sediments were being shed off the rising Rocky Mountains into the subsiding Denver Basin east of the mountains.

The lower exposed facies of the Dawson is a “greenish-gray to olive-brown pebbly sandstone composed almost exclusively of andesitic material” interbedded with siltstone and sandy claystone.  Locally, near the base of the andesitic unit, lenticular beds of pebbly sandstone and/or conglomerate, and sandy claystone representing reworked beds of the older Pierre Shale and Fox Hills Sandstone are exposed (Thorson and others, 2001).  This lower unit forms the vegetated slopes at the base of the Pulpit Rock cliffs.

Andesite is a gray, fine-grained volcanic rock, with a high percentage of plagioclase feldspars (named after rocks well exposed in the Andes Mountains); therefore, andesitic sediments contain a high percentage of fragmental volcanic andesite.  However, when one examines the current mountains west of Colorado Springs large exposures of andesite are essentially absent!  The question then becomes---what was the source of these sediments?  Robert Raynolds and Kirk Johnson from the Denver Museum of Nature and Science have studied extensively the sediments and rocks of the Denver Basin and believe this episode of sedimentation represents “uplift of a portion of the Front Range bounded by the Golden and Rampart Range faults…[where] andesitic volcanic rock that covered much of the Front Range was stripped from the uplift and deposited” in the Basin (Raynolds, 2002).  The Rampart Range fault forms the eastern boundary of the Rampart Range near Colorado Springs.  Thorson and others (2001) noted that large boulders (up to three feet) and logs (up to eight feet in length) in the andesitic sediments indicate deposition in a very energetic stream environment.  All of this evidence points to a large system of overlapping braided streams and fans radiating off the rising Rampart Range, stripping off the andesite, and depositing the resulting sediments to the east of the mountains.  This is a great example of sediments and rocks informing geologists about the past presence of volcanoes to the west of Colorado Springs although physical remnants (the landforms) of these volcanoes are now absent.

The upper unit of the Dawson exposed at Pulpit Rock is mostly a white to light-gray, crossbedded to massive, coarse-grained arkosic (feldspar-rich) sandstone or pebble conglomerate.  Locally there are gray, massive mudstones representing ancient mudflows, and brownish-gray, organic-rich siltstones to claystones representing deposition in ephemeral swamps (Thorson and others, 2001).  This upper unit forms the massive cliffs and spires of Pulpit Rock proper.

There is a distinct change in composition of the rocks forming the lower unit of the Dawson from rocks in the upper unit.  While the lower unit is composed of andesitic material as previously described, the upper unit is almost devoid of andesitic sandstone.  Instead of the gray plagioclase feldspar common in the lower unit, the upper sandstones have a high percentage of pink feldspars characteristic of a granitic source terrain.  This arkosic sandstone would seem to indicate that that a “wedge or lobe of andesite-free debris was shed eastward from a granitic source along the western edge of the basin” (Madole and Thorson, 2003).  Perhaps enough of the Paleozoic and Mesozoic rocks had been stripped from the rising Front Range so as to expose the Precambrian Pikes Peak Granite?

Although the Dawson Formation spans the K-T Boundary (~65.5 my) in the Denver Basin, the rocks exposed at Pulpit Rock are latest Cretaceous in age at approximately 66 my (Johnson, personal communication, 2008).  Vertebrate fossils, including dinosaurs, have been collected at other localities in the Dawson Formation; however, bones seem rare or non-existent at Pulpit Rock.  On the other hand, researchers from the Denver Museum of Nature and Science (with appropriate collecting permits) have excavated numerous late Cretaceous leaves and other plant material from the Park.  In addition, any causal hike through the Park reveals numerous pieces of petrified wood scattered about on the surface.

It has been common practice in past years to assign the lower andesitic unit of the Dawson Formation to the Denver Formation and one still hears that term used.  However, Madole and Thorson (2003) have shown that the Denver Formation of the north pinches out and intertongues with the Dawson near Colorado Springs.  In addition, the source areas for the Denver Formation and the andesitic facies of the Dawson Formation are different.  Therefore, the use of the name Denver Formation for rocks at Colorado Springs is invalid.

The hike to the summit of Pulpit Rock is invigorating and geologically interesting.  In addition to the exposures of the Dawson Formation, one has a great view of Pikes Peak and Cheyenne Mountain, the Rampart Range, Monument Creek, Rockrimmon, and Popes Bluff/Popes Valley.  Time your hike in the evening and the sunset is spectacular.

      REFERENCES CITED

 Madole, R.F. and Thorson, J.P. Geologic Map of the Elsmere Quadrangle, El Paso County, Colorado. Denver: Colorado Geological Survey. Open-File Map and Report 02-2, 2003.

Raynolds, R.G. 2002, Upper Cretaceous and Tertiary Stratigraphy of the Denver Basin: Rocky Mountain Geology, v. 37, no. 2. 

Thorson, J.P., Carroll, C.J. and Morgan, M.L. Geologic Map of the Pikeview Quadrangle, El Paso County, Colorado. Denver: Colorado Geological Survey, Open-File Map and Report 01-3, 2001.

2 comments:

  1. Hello Mike, I've enjoyed following your post for a couple years, myself I enjoy not just rock hounding but trying to figure out the past geology of Colorado springs. I've come across some interesting rocks and if like your imput on top what they are. I'd send you a pic but don't know how to do it on this forum. If I could get your email that would be great. I believe I've found a sort of shocked quartz up on pulpit rock and some other interesting things. Also I have a theory on the geology and geography of this area I'd like to share.

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  2. You need to send info to csrockboy at yahoo dot com

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