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M E E T I N G   N O T E S

Update of Tom Steven/ Dan Shawe Geomorphic Work (Geologic Evolution of Kenosha-Tarryall Landscape)

Speaker: Tom Steven

March 20, 2005

 Location: Woodland Park Library



 The popular concept of THE Late Eocene erosion surface in the region is grossly misleading.  The presence of a Late Eocene unconformity in places is not disputed, especially where cover rocks date this surface, but a lot of surfaces labeled as such are probably younger in origin, later than Oligocene volcanic activity.  There are apparently several Miocene (and sometimes later) erosional events in response to local tectonic hiccups.  These are collectively referred to in terms of a generic Miocene surface.  In detail, these somewhat correlative surfaces are not everywhere the same age.  A major regional complexity relates to the fact that Miocene and perhaps younger erosion has apparently stripped off large areas of Oligocene volcanic rocks, so dating becomes extremely challenging.

 Post-volcanic erosion is therefore the basis for rather detailed mapping using USGS 7 1/2 minute quads. Ovoid to more irregular, mosaic-faulted protrusions characterized by remnants of upland rolling topography and locally preserved deep weathering profile can be recognized widely in the region where >13000 ft mountains reside.   This style includes parts of Pikes Peak, Mt. Bierstadt/Evans, Fremont Pass, Rocky Mountain Nat’l Park, Central City area, and the northern end of Sawatch Range (Mt Massive, Elbert).  In fact, this surface can be traced over large geographic areas of the southern Rockies.

 Why are so many peaks clustered in the 13-14,000+ range?  This is probably a result of gravity/ Archimedes principle, whereby the contrast between crustal vs. deeper density rocks allows high-standing blocks to “float” about the same level.

 There has been considerable post-Oligocene jostling of blocks in the region, apparent from interruption of the generic Miocene surface.  Significant grabens developed, along with clastic debris aprons/debris flows.  The Ogallala formation of the plains records the sediment shed from the high protrusions, etc. during the Miocene.

 The ovoid protrusions generally are dissected by a very different geomorphic style of incised terrain.  This style records complex canyon cutting events containing discrete, mappable drainage systems. Stream piracy is sometimes apparent, as well as later tilting of local drainages. 

 The Kenosha-Tarryall region is a large area of varied styles, including an ovoid protrusion, a sub-circular dome (McCurdy Mtn), incised topography, hanging valleys, shattered terrain, etc.  Especially intriguing is the Lost Creek/Goose Creek trend, consisting of a series of discrete slide blocks probably representing a young zone of major seismicity.  (secretary’s note: this NNW-SSE trend lies along the northern portion of the Florissant Lineament, a long-lived fault responsible for many geologic anomalies in the region).

 Some other areas analyzed also display apparently very young tectonics, including Bailey-Grants and South Park.  Regarding the latter, the Fairplay West quad shows “indisputable evidence of young faulting”.  The Elkhorn thrust, which may imbricate on its north end, appears to have been swamped by recent fault scarps also.

 This raises the question: how young is young regarding the regional tectonics?  Are these recent episodes just a continuation of Miocene tectonics, or is there a separate but significant post-Miocene (possibly post-Pliocene) tectonic event?