Odd rocks and the New Catastrophism
John E. Warme (‘59), Professor Emeritus, Department of Geology and Geological
Engineering, Colorado School of Mines, Golden, Colorado, 80401
“ONE MUST UNDERSTAND THE EXPECTABLE
IN ORDER TO RECOGNIZE THE ANOMOLOUS.”
Most dedicated geologists are curious about rocks of all kinds. With
experience we learn what to expect when encountering rocks of different ages and
in new places. Such background knowledge forms a basis for recognizing rocks
that so not seem to belong where they are found. Investigating these geologic
anomalies leads to new knowledge and perhaps spectacular revelations. Three
very different examples, from my experience, illustrate how recognition of
anomalous rocks resulted in new interpretations. All three involve major
cataclysmic geologic events that had previously gone unrecognized:
1. Giant Olistolith/Olistostromes, Jurassic, Morocco: At one location in the High
Atlas Mountains of Morocco, Jurassic corals were formerly interpreted to have
been shed from an anomalous, shallow-water, mid-rift ridge. Based on new
fieldwork, regional relationships, and recognition of critical sedimentary
structures, the ridge was reinterpreted as a 30-km-long massive olistolith of
cemented rift-margin carbonate facies that slid into deep water. The corals were
shed from the newly formed scarp, far away, as voluminous olistostromes.
2. Alamo Impact Breccia, Devonian, Nevada: Twenty five or more mountain
ranges in and around southeastern Nevada contain Upper Devonian breccias in
the cyclic shallow-water carbonate platform beds of the Guilmette Formation. The
broken rocks were variously interpreted as coral-reef talus, karst cave-fillings, and
fault or solution-collapse breccias. Regional field relationships revealed that many
of these breccias, some as much as 100+m thick, belong to a single correlative
interval that was deposited ~382 million years ago. The breccias were created by
a large-scale Late Devonian marine bolide, targeted about 150 km north of the
present city of Las Vegas.
3. Pancho’s Radical Runup, Pleistocene, Grand Canyon: In the Grand Canyon,
Arizona, two distinctive Cambrian dolostone members are interbedded within the
Bright Angel Shale and weather to a distinctive rusty-brown color. For 1.5 km
along the south Canyon wall, five patches of broken rock were mapped as
travertine-cemented talus and small-scale landslides. New fieldwork showed that
the patches are comprised of the same distinctive dolostones, but anomalously lie
across and ABOVE their in situ sub-horizontal layered position. The patches are
the erosional remainder of a single cataclysmic detachment that slid from the
north side of the canyon and flowed 300+m up the south slope. These rocks have
been seen by countless geologists and others, but not recognized as a massive
landslide and the highest cross-canyon runup recognized in the 48 contiguous
United States.