Rocky Mountain Geology 35.2

Mesoproterozoic reactivation of a Paleoproterozoic transcurrent boundary in the northern Colorado Front Range: Implications for ~1.7- and 1.4-Ga tectonism

Jane Selverstone, Meghan Hodgins, John N. Aleinikoff, and C. Mark Fanning

Keywords: Tectonic reactivation, Mesoproterozoic, Paleoproterozoic, Front Range, Colorado, transcurrent faulting

Proterozoic metamorphic and igneous rocks in the northern Colorado Front Range display evidence for significant but localized 1.4-Ga deformation. Initial metamorphism and deformation occurred during crustal assembly ~1.7 Ga, and the area was subsequently affected by widespread "anorogenic" granitic plutonism at ~1.4 Ga. Although there is little evidence for penetrative 1.4-Ga deformation in northern Colorado, data from the northeast-striking Moose Mountain shear zone indicate localized 1.4-Ga contractional strain. The shear zone deforms both pre-1.70-Ga supracrustal rocks and the ~1.4-Ga St. Vrain granite. Kinematic indicators within the deformed granite show south-side-up reverse motion. A continuum from magmatic to solid-state mylonitic fabrics indicates that deformation occurred during the emplacement and cooling of the granite at ~1.4 Ga. Thrust-sense deformation, coupled with synchronous northeast–southwest extension recorded in dike swarms, is consistent with a model for regional northwest-directed contractional deformation during emplacement of the 1.4-Ga plutonic suite. Paleoproterozoic supracrustal rocks within the shear zone show evidence for the 1.4-Ga shearing as well as for an older phase of deformation apparently dominated by sinistral motion. Supracrustal rocks on opposite sides of the shear zone record different structural histories, and detrital zircon samples differ dramatically in age populations across the shear zone. These data lead us to suggest that the shear zone separates two unrelated packages of rocks that were juxtaposed against one another during ~1.7-Ga assembly of the region. Thus, original assembly of this region around 1.7 Ga probably involved previously undocumented transcurrent movements that juxtaposed "terranes" with differing Paleoproterozoic histories and resulted in large-scale zones of crustal weakness that localized subsequent deformation.

Amber from Upper Cretaceous through Paleocene strata of the Hanna Basin, Wyoming, with evidence for source and taphonomy of fossil resins

David A. Grimaldi, Jason A. Lillegraven, Thomas W. Wampler, Denise Bookwalter, and Alexander Shedrinsky

Keywords: Amber, cone scales, Cretaceous, Hanna Basin, paleobotany, Paleocene, Pinaceae, PyGC-MS, taphonomy, Taxodiaceae, Wyoming

The Hanna Basin is a relatively small foreland basin in south-central Wyoming containing a combined thickness of roughly 38,000 ft (11.5 km) of Upper Cretaceous and Paleocene strata. Amber occurs in the Hanna Basin in carbonaceous to lignitic strata, representing fluvial and paludal episodes bounded by incursions of epicontinental seas. Amber occurs, in decreasing age, in the Upper Cretaceous Allen Ridge, Medicine Bow, and Ferris formations (parts of the last straddle the Cretaceous–Tertiary boundary), as well as in the Paleocene Hanna Formation. Because of the extraordinary thickness, unequivocal stratigraphic superposition, and long-lived deposition of Upper Cretaceous and Paleocene amber-bearing strata in the Hanna Basin, a unique opportunity has been provided for integrated study of taxonomic sources, deposition, and taphonomic alteration of ancient resins.

In all relevant Cretaceous and some Paleocene outcrops the amber is preserved mostly as small (4–8 mm diameter) droplets, often highly weathered and oxidized. One site in the Hanna Formation has yielded abundant, large pieces of transparent amber. Composition of samples analyzed by pyrolysis/gas chromatography-mass spectroscopy (PyGC-MS) indicates a common taxonomic source for amber from the Allen Ridge, Medicine Bow, and Hanna formations. The taxonomic source of amber from one part of the Ferris Formation, in contrast, is unique among the sites sampled; its chemical signature probably reflects a distinctive paleoenvironment and flora, originally recognized through palynomorphs. The characteristic PyGC-MS profile from that site is highly indicative of the Dipterocarpaceae, which would imply a rare but expected Mesozoic record of amber from a dicotyledonous tree.

In the Hanna Basin a stratigraphic interval of more than 5 mi (>8 km) and a time gap of approximately 20 million years separate the lowest and highest occurrences of amber. Such a range in one stratigraphic sequence is unprecedented among known deposits of amber. Of particular interest is that most of these samples apparently were formed by one or several closely related species of trees. The amber is chemically and physically mature, no doubt due to deep burial. Nevertheless, despite dramatic differences in age and depth of burial, only minor chemical changes from diagenetic causes were detected among the samples. Inclusions in well-preserved pieces of amber from the Hanna Formation are fairly abundant, but typically they are distorted or were partially destroyed by effects of compaction and/or microscopic-scale deformation. Sparse wood and plant fragments and spores/pollen grains are present, but only one insect (a thrips: Order Thysanoptera) has been recognized.

Distinctive scales of conifer cones occur in the Allen Ridge Formation. The scales contain radiating vessels of resin, and they represent the taxonomically equivocal genus "Dammara." PyGC-MS analysis of the vessel resin indicates that the same kind of tree that produced these cone scales also produced the amber in the Allen Ridge, Medicine Bow, and Hanna formations. Moreover, chemical composition of these samples closely matches that from vessels of "Dammara" cone scales from Upper Cretaceous (Turonian) strata in eastern North America. Circumstantial association of "Dammara" cone scales with several types of fossilized foliage suggests Taxodiaceae as the common source, although wood anatomy and amber chemistry also suggest Pinaceae. In spite of this taxonomic uncertainty, it is probable that 30 million years of amber production during the Late Cretaceous and Paleocene in northern North America, and probably much of Holarctica, was the result of a genus of tree that produced "Dammara" cone scales. These new data cast serious doubt upon recent proposals that all Cretaceous ambers were formed by members of the Araucariaceae. Wax residues were chemically discerned in one specimen of cone scale.

Cenozoic tectonic evolution of the Ruby Mountains metamorphic core complex and adjacent valleys, northeastern Nevada

Peangta Satarugsa and Roy A. Johnson

Keywords: Seismic reflection, seismic refraction, metamorphic core complex, Ruby Mountains, Nevada

Seismic-reflection and borehole data along with crustal-scale refraction/reflection data provide new evidence for the Cenozoic tectonic evolution of the Ruby Mountains metamorphic core complex and Huntington, Ruby, and Lamoille valleys. Analyses of these data suggest: (1) along the western flank of the Ruby Mountains an early stage of upper-crustal extension provided accommodation space for deposition of apparently synextensional strata; (2) the oldest sedimentary rocks in the developing basin along the western flank of the Ruby Mountains are middle Eocene in age, suggesting that active upper-crustal extension and early basin formation in northeastern Nevada began at least by that time; (3) Ruby Valley is bounded by high-angle, east-dipping normal faults on the west and a relatively low-angle, west-dipping normal fault on the east; (4) crustal thicknesses beneath the eastern flank of the Ruby Mountains do not reflect local topographic relief and estimated amounts of extension; and (5) adjacent to the range, maximum thicknesses of basin-fill sedimentary rocks do not directly reflect maximum amounts of exhumation of the Ruby Mountains. Together, these observations suggest that either pre-existing crustal roots (subsequently dissipated), or middle- or lower-crustal flow prior to and during extension, were involved in evolution of the core complex.