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

Abstract

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 Palecene 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.