VOLUME 39 NUMBER 2
Timing, style, and significance of Cambrian through Laramide brittle reactivation along the Proterozoic Homestake shear zone, Colorado mineral belt
Joseph L. Allen
Detailed mapping and stratigraphic studies at the
intersection of the Proterozoic Homestake shear zone with the early
Paleozoic section exposed on the margins of the Laramide Sawatch
anticline in central Colorado provide limits on the timing and
magnitude of brittle reactivation during Phanerozoic time. An episode
of displacement rooted within two distinct ductile branches of
the Homestake shear zone in the northeastern Sawatch Range generated
an 8-km-wide, Late Cambrian fault block that had 420 m of
paleotopographic relief prior to depositional onlap by sandstones of
the Sawatch Formation. An episode of up-to-south displacement (<30 m)
subsequently occurred along the southern part of the shear zone in
the northeastern Sawatch Range and in the western Sawatch Range
during deposition of the Lower Ordovician Manitou Formation. A third
episode of reactivation during Early to Middle Ordovician time
(post-Manitou Formation, pre-Harding Sandstone) resulted in renewed,
decameter-scale uplift of the Late Cambrian fault block in the
northeastern Sawatch Range. A fourth episode of reactivation during
Late Cretaceous Laramide deformation produced localized strike-slip
displacement along brittle faults on the northeastern flank of
the Sawatch Range. The three early Paleozoic episodes of reactivation
occurred on a cratonic platform and are genetically linked to
extension and thermal uplift during intrustion of a suite of bimodal,
rift-related plutonic rocks (Iron Hill and Wet Mountains intrusive
rocks). They were emplaced 
135 km south
of the Homestake shear zone along the Cimarron-Red Rocks fault
and Apishapa fault system. The reactivation history model proposed
herein differs from some previous interpretations that cumulatively
suggest at least eleven episodes of Phanerozoic reactivation,
including relatively large-magnitude late Paleozoic displacement.
Key Words: Fault reactivation • intracratonic deformation • tectonic heredity • Southern Rocky Mountains • Homestake shear zone • Cambrian • Ordovician • Colorado • Sawatch Range • Sawatch Formation • Dotsero Formation • Manitou Formation
The Wyoming Jurassic fossil Dentalium subquadratum Meek, 1860 is not a scaphopod but a serpulid worm tube
C. Philip Palmer , Donald W. Boyd
and Ellis L. Yochelson
Since 1860, the record of Jurassic scaphopod mollusks has included Dentalium subquadratum from Wyoming. Restudy of F. B. Meek's type material indicates that the small, arcuate, tapered tubes lack definitive scaphopod characters. Instead, they resemble shells built by serpulid worms (Polychaeta) of the genus Hamulus, so D. subquadratum Meek, 1860 is here transferred to Hamulus subquadratus (Meek, 1860). Fieldwork indicates that Meek's syntype slab came from the Redwater Shale Member (Oxfordian) of the Sundance Formation, 14 mi (22.5 km) southwest of Casper, Wyoming.
Key Words: Dentalium • F. B. Meek • F. V. Hayden • Hamulus • Jurassic • Oxfordian • serpulid worm • Sundance Formation • Wyoming
Stratigraphy and depositional environments of the upper Fox Hills and lower Hell Creek Formations at the Concordia Hadrosaur Site in northwestern South Dakota
Mary C. Colson , Russell O. Colson
and Ron Nellermoe
Many of the dinosaur-bearing bone beds in the Hell Creek Formation of the Dakotas and Montana involve multiple species preserved in the upper Hell Creek Formation. In contrast, the Concordia Hadrosaur Site is monospecific with respect to dinosaurian taxa and is situated in the lower Hell Creek Formation in a lithostratigraphic unit we associate with the Little Beaver Creek Member. This member consists of organic-rich sandstones, siltstones, and claystones that are distinctive within the Hell Creek Formation based on their uniformly fine grain size, purplish color, and presence of highly lignitic shale rather than coal. Similar lignitic deposits occur at other marine-terrestrial boundaries of the Fox Hills-Hell Creek Formations in the Little Missouri and Missouri River valleys. The bone bed at the Concordia Hadrosaur Site (CHS) is associated with an extensive coastal swamp rather than a localized fluvial subenvironment such as river channel, floodplain, or abandoned channel. The bone bed itself lies at the transition from an extensive swamp (represented by highly organic mudstones) to a more fluvially dominated, distributary environment characterized by variegated mudstones, siltstones, and channel sandstones. The thirty meters of exposed section at the Concordia site include the top of the Fox Hills Formation and lower parts of the Hell Creek Formation. We identify marine silts, muds, and sands, coastal dune sands, coastal swamp muds and silts, and fluvial sands and silts. The sediments are indicative of the marine-terrestrial transition from upper shoreface and foreshore environments to a complex system of coastal dunes, swamps, and distributary channels that formed during the progradation of the Hell Creek sediments into the Cretaceous Fox Hills seaway. Locally, grain size and organic fraction varied due to differences in the proximity to distributary channels, supply of organic material, and water depth. Despite the concentration of bones dominated by a single species in the CHS bone bed, the high clay fraction of the bone bed matrix, combined with the fact that the lowest part of the bone bed has the greatest clay fraction, indicates that the bones were not introduced by way of a high-energy, catastrophic event, such as a flood. Rather, the bones accumulated in an area of quiet standing water. Although preliminary examination of the bones is consistent with this depositional interpretation, it does not necessarily provide direct support for it.
Key Words: coastal progradation • Cretaceous • Edmontosaurus • Fox Hills Formation • Hell Creek Formation • Maastrichtian • North Dakota • South Dakota
Unconformities and age relationships, Tongue River and older members of the Fort Union Formation (Paleocene), western Williston Basin, U.S.A.
Edward S. Belt , Joseph H. Hartman ,
John A. Diemer , Timothy J. Kroeger , Neil E.
Tibert and H. Allen Curran
An unconformable relationship is observed within the Paleocene
Fort Union Formation in the western Williston Basin at the contact
between the Tongue River Member and the underlying Lebo and Ludlow
Members. Isotopic dates and pollen biozone data reported here are
integrated with previously published data. A new correlation of these
facies results in a revised history of localized depositional and
tectonic events. One unconformity occurs at this lithological contact
in the Pine Hills (PH), Terry Badlands (TB), and Ekalaka (E) areas
west of the Cedar Creek anticline (CCA), and another unconformity
occurs at the same lithological contact in the Little Missouri River
(LMR) area east of the CCA. The two unconformities differ in age by
about two million years. The older is the U2 and the
younger is the U3, which initially were recognized in the
Ekalaka area of southeastern Montana (Belt et al., 2002). The U2
crops out in the TB, PH, and E areas, where at least 85 m of Tongue
River strata bearing palynomorphs characteristic of biozone P-3 are
found above the unconformity. Radiometric dates from strata (bearing
palynomorphs characteristic of biozone P-2) below the U2
range in age from 64.0 to 64.73 Ma. The U2 unconformity
west of the CCA thus occurs in strata near the base of the lower P-3
biozone. The U3 crops out in the LMR area (east of the CCA), where only
13 m of strata characterized by the P-3 pollen biozone occur
above it. Radiometric dates from an ash <1 m above the U3
in that area range in age from 61.03 to 61.23 Ma, and the P-3/P-4
pollen biozone boundary is located 13 m above the ashes. The U3
thus occurs in strata characterized by upper parts of the P-3 pollen
biozone east of the CCA. The U3 is also identifiable in
the middle of the ca. 200 m-thick Tongue River Member west of the CCA,
where mammal sites 40 to 80 m above it are Tiffanian-3 in age. The
strata below this unconformity are tilted gently to the northwest;
strata above the unconformity are flat lying. This mid Tongue River
unconformity probably correlates with the unconformity at the base of
the Tongue River Member in the LMR area east of the CCA, where a Ti-2
mammal site (the "XX" locality) occurs <10 m above it.
Depositional and tectonic events can be summarized using North
American Mammal Age nomenclature as a relative time scale. From
latest Cretaceous through Puercan time, paleodrainage was toward the
east or southeast, in the direction of the Cannonball Sea. The Black
Hills did not serve as an obstruction at that time. During early
Torrejonian time, the Miles City arch (MCA) and Black Hills were
uplifted and partially eroded, leading to the U2
unconformity. When deposition resumed, paleodrainages shifted to a
northeasterly course. During middle and late Torrejonian time, facies
of the lower Tongue River ("Dominy") sequence and the Ekalaka Member
of the Fort Union Formation were deposited in the middle of a
subbasin between the MCA and the CCA. Simultaneously, smectite-rich
components of the Ludlow Member were being deposited east of the CCA.
During latest Torrejonian time, uplift of the Black Hills tilted the
"Dominy" sequence toward the northwest and local erosion led to the U3
unconformity. Following this tilting, during Tiffanian time,
deposition of the upper Tongue River ("Knobloch") sequence shows
continuity from western North Dakota across eastern Montana and into
the northern Powder River Basin.
Key Words: mammal ages • Paleocene • pollen biozones • radioisotopic ages • stratigraphy • tectonics • unconformities • Williston Basin
The King and Hayden Surveys: Overlap of pioneering mapping in northern Colorado
K. R. Aalto
Mapping by two of the great post-Civil War geological surveys of the American West overlapped in several areas, including a strip across northern Colorado [King and Hayden Surveys]. Prior to the publication of atlases [King1876, Hayden1877] geologists of these surveys exchanged little information, yet their final maps in areas of overlap are fairly similar. King's construction of topographical maps to serve as the basis for portraying geology was adopted by the other surveys. While the degree of detail shown and age interpretation of some stratigraphic units vary between the maps, overall geologic trends are similar. In 1878, King justified his survey expenses, writing that: "This exploration has not duplicated other surveys made by authority of Congress." While overlap of map areas in part reflects varied goals of survey leaders and/or imprecisely defined survey and territorial boundaries, it also reflects the strong personalities and rivalry among these men and their unwillingness to share information. This set the stage for the consolidation of western surveys under the authority of a single agency, the newly formed U. S. Geological Survey with Clarence King as its first director.
Key Words: Clarence King • Ferdinand Vandeveer Hayden • Great Surveys of the West • Uinta Mountains • Colorado