Day 3
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Field School, Day 3 (7/8/03) - Plum Creek and Pulpit Rock, Colorado Springs

On Day #3, we began at 8:00 a.m. with a discussion full of definitions (see notes below).  There is great confusion in the Denver Basin due to past nomenclature provided for the various sedimentary layers and geographic areas.  Different people use Arapahoe, Dawson & Denver names to refer to different sections, or even the same section. The aquifer names further complicate naming because they use the same names, but these refer to depths which have nothing to do with lithology.  It is important to remember that time does not equal sediments - during a given time, different sediments are being deposited depending on their source rock.  So, naming a formation in the Denver Basin based on rock type doesn't work.

In Colorado Springs, the basal material is actually Pierre Shale that rode up with the mountain uplift.  Then, erosion deposited the Pierre material (such as shark teeth and phosphate) back into the basin.  Thus, most of the Pierre shale material and fossils in this area are "reworked" material which only preserves the hardest materials such as shark teeth.

We visited a location on Monument Creek near Mark Dabling Rd.  It is a braided stream which flows southward towards the Arkansas river.  We were immediately presented with a nice cut bank displaying silty mudstones, coal/lignite beds, overlying mudstones and more recent dirt.

At the next stop, we visited the base of Pulpit Rock in Colorado Springs.  It's dated at approximately 68 mya which places it in the late Cretaceous.  It contains the Pierre Shale, small pebbles, petrified wood, dinosaur bones and dinosaur footprints.  We were instructed to look carefully at the pebbles because they are "business cards of the source material."  In doing so, we were not disappointed.  In a matter of minutes, the group found a number of shark teeth and bones of the Cretaceous.

Then, we experienced an argument between scientists.  Some may be disconcerted by what took place, but this is truly the way science is revealed.  After considerable discussion, digging, explanation and argument - one side prevailed.  In the meantime, a new fossil plant locality was discovered which was aptly named "Lost Argument".  It's believed to be approximately 68 mya at the base of D1.

We were searching for a Cretaceous fossil leaf site near the mountains on this expedition.  These are rare due to the slower deposition of sediments which results in burrowing by small creatures and destruction of fossil leaf material.  We were successful in excavating a previously known location called the "Zebra Beds".  It is a thinly laminated area of alternating colors which contains Cretaceous leaves.  At this site we found approximately 20 different leave species including pinnate leaves, palms and ginko.

The base of Pulpit Rock is an enigma.  It contains pink feldspar, black quartz, arkose, etc.  However, just above the base is an unusual slurry of debris flow.  It appears to be from a landslide/mudslide event, but it is still unexplained.  For more on this, see the pictures link at the bottom of this page.


Arkosic: ground up granite (quartz and feldspar)

Andisitic: dark green/brown material of a volcanic origin.

Dawson Arkose: named after the butte S/W of Denver.  Whole hill is arkose, but late Cretaceous to early Eocene.

Denver Formation: andisitic sand, volcanic sand and conglomerate. 


Images from Day 3