Sidney R. Ash

A review of selected triassic to Early Cretaceous ferns

After becoming nearly extinct during the Permian, the ferns began a slow recovery during the Triassic as the climate of the earth moderated. As a result, a considerable number and variety were present and widely distributed during the Jurassic and Early Cretaceous. However, with the rapid expansion of the angiosperms during the Late Cretaceous, the ferns once again became reduced in variety and greatly restricted in distribution. Some of the Mesozoic ferns are rather primitive and obviously are closely related descendants of Paleozoic taxa. Such ferns are assigned mostly to the Marattiaceae, Guaireaceae, Osmundaceae, and Gleicheniaceae. The majority of the Mesozoic ferns, however, are distinctive and appear to have originated during that era. These fossil ferns generally fit into modern orders and families such as the Matoniaceae or the Dipteridaceae. In some cases, it is difficult to clearly distinguish some of the Mesozoic ferns from living genera.

Preliminary palynological zonation of the Chinle Formation, southwestern U.S.A., and its correlation to the Newark Supergroup (eastern U.S.A.)

Abstract Three informal palynological assemblage zones can be distinguished in samples from Chinle Formation outcrops in Utah, Arizona and New Mexico. The oldest zone (zone I) is in the Temple Mountain Member in southeastern Utah; the middle zone (zone II) is in the Shinarump, Moss Back, Monitor Butte and (lower part of the) Petrified Forest Members (Utah, Arizona and New Mexico); the youngest zone (zone III) is in the upper Petrified Forest Member and silstone member in Arizona and Utah and the silstone member in northcentral New Mexico. Present palynological evidence suggests that Chinle deposition on the Colorado Plateau began locally in late Carnian time and continued at least into the early part of Norian time of the Late Triassic period. Because the upper boundary of the Chinle Formation is an unconformity and the overlying formations are palynologically barren, the length of time represented by this stratigraphic hiatus is not known with certainty. Current palynological evidence suggests, however, that the unconformity at the top of the Chinle cannot be older than early Norian nor younger than Hettangian. Zones I, II and III can now be recognized in the palynomorph assemblage sequences from the Eastern Mesozoic basins, which modifies earlier palynological zonations for the lower portions of the Newark Supergroup. This is based on our identification of palynomorphs not previously known from portions of the Newark Supergroup and the discovery that specific biomarker taxa combinations are the same for both the western and eastern palynomorph sequences. At present palynomorph assemblages from the Chinle Formation and Newark Supergroup compare more closely for zones II and III than they do for zone I, but research is still in progress.

Isotopic age of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona: An example of dating a continental sandstone

Zircons from the Black Forest Bed, Petrified Forest Member, Chinle Formation, in Petrified Forest National Park, yield ages that range from Late Triassic to Late Archean. Grains were analyzed by multigrain TIMS (thermal-ionization mass spectrometry), single-crystal TIMS, and SHRIMP (sensitive, high-resolution ion-microprobe). Multiple-grain analysis yielded a discordia trajectory with a lower intercept of 207 6 2 Ma, which because of the nature of multiple-grain sampling of a detrital bed, is not considered conclusive. Analysis of 29 detrital-zircon grains by TIMS yielded UPb ages of 2706 6 6M a to 2066 6 Ma. Eleven of these ages lie between 211 and 216 6 6.8 Ma. Our statistical analysis of these grains indicates that the mean of the ages, 213 6 1.7 Ma, reflects more analytical error than geologic variability in sources of the grains. Grains with ages of ca. 1400 Ma were derived from the widespread plutons of that age exposed throughout the southwestern Cordillera and central United States. Twelve grains analyzed by SHRIMP provide 206 Pb*/ 238 U ages from 214 6 2M a to 200 6 4 Ma. We use these data to infer that cores of inherited material were present in many zircons and that single-crystal analysis provides an accurate estimation of the age of the bed. We further propose that, even if some degree of reworking has occurred, the very strong concentration of ages at ca. 213 Ma provides a maximum age for the Black Forest Bed of 213 6 1.7 Ma. The actual age of the bed may be closer to 209 Ma. Dating continental successions is very difficult when distinct ash beds are not clearly identified, as is the case in the Chinle Formation. Detrital zircons in the Black Forest Bed, however, are dominated by an acicular morphology with preserved delicate terminations. The shape of these crystals and their inferred environment of deposition in slow-water settings suggest that the crystals were not far removed from their site of deposition in space and likely not far in time. Plinian ash clouds derived from explosive eruptions along the early Mesozoic Cordilleran margin provided the crystals to the Chinle basin, where local conditions insured their preservation. In the case of the Black Forest Bed, the products of one major eruption may dominate the volcanic contribution to the unit. Volcanic detritus in the Chinle Formation was derived from multiple, distinct sources. Coarse pebble- to cobble-size material may have originated in eastern California and/or western Arizona, where Triassic plutons are exposed. Fine-grained detritus, in contrast, was carried in ash clouds that derived from caldera eruptions in east-central California or western Nevada.

Palaeoclimatic interpretation of the wood structures of the trees in the Chinle Formation (Upper Triassic), Petrified Forest National Park, Arizona, USA

Abstract Typically, the fossil woods in the Upper Triassic Chinle Formation in Petrified Forest National Park, Arizona, USA do not show annual growth rings but contain irregular growth interruptions similar to those found in trees now growing in the humid tropics. These interruptions could be due to endogenous hormonal effects or to occasional local variations in water supply Therefore, it is conclucded that the Late Triassic fossil forests in the park lived in conditions that permitted continuous tree growth. Limited data from areas outside of the park suggests that similar conditions prevailed in adjacent areas in the southwestern United States during the Late Triassic. Although this region is reported to have been under the influence of a monsoonal circulation pattern during the Late Triassic, the growth of the trees does not appear to have been greatly affected.

Late Triassic charcoal from Petrified Forest National Park, Arizona, USA

Fossil charcoal has been found in the Late Triassic Chinle Formation in Petrified Forest National Park, a location that is world famous for its silicified tree trunks. The material consists of charcoalified secondary wood, and has sufficiently well preserved plant anatomy to show it may be described as ‘araucarian type’, although it does display some minor differences from the anatomies described for the silicified gymnosperm trunks in the park. The presence of the charcoal proves that on occasion natural wildfires, most probably ignited by lightning, must have occurred in the Chinle Triassic palaeoenvironments, with resulting impacts on the biota of those times. It is currently impossible to estimate how often these fires occurred, as the incidence of charcoal here and in the whole fossil record apart from the frequency of ancient fires also reflects the preservation and burial conditions of the material. There is scant mention of Triassic charcoal in the literature, a notable absence given the abundance of charcoal that has been recorded in the Carboniferous, Jurassic, and Cretaceous periods. This lack of charcoal may support the theory that atmospheric oxygen levels were lower during the Triassic than in the Carboniferous.

EVIDENCE OF ORIBATID MITE HERBIVORY IN THE STEM OF A LATE TRIASSIC TREE FERN FROM ARIZONA

Abstract Small coprolite-bearing borings occur in the stem of the filicalean tree fern Itopsidema vancleaveii Daugherty from the Chinle Formation of Late Triassic Age (Carnian Stage) in Petrified Forest National Park, Arizona. These borings are restricted to parenchyma within the leaf petioles and among the adventitious roots of the root mantle. Although they are not lined by wound tissue, some of the borings in the leaf petioles contain small discontinuous masses of wound-tissue at a few places along some of the walls, indicating that the plant was alive when it was attacked. Coprolites within the borings generally are small (mostly about 40–50 μm in diameter and 85–100 μm in length) and oval in longitudinal section and round to weakly hexagonal in transverse section; they consist of very small particles of unidentifiable plant matter. Although the weakly hexagonal coprolites are similar to those produced by termites but they are an order of magnitude smaller. Furthermore, the borings are much smaller than those produced by known extant termites. It is likely that oribatid mites produced the coprolite-bearing borings and coprolites. This occurrence is significant because it bridges the Late Permian to Early Jurassic gap in the geologic record of endophagous mites and also contributes new data on arthropod activity during the Late Triassic in southwestern North America.

Evidence of widespread fungal attack on Upper Triassic trees in the southwestern U.S.A.

Abstract Certain Upper Triassic tree trunks in the southwestern U.S.A. show evidence of damage similar to that caused in tree trunks at the present day by pocket rot fungi such as Polyporus amarus Hedgc. and Heterobasidion annosum (Fries) Bref. The damaged trunks occur in a relatively thin stratum at the same horizon over a wide area in the Petrified Forest National Park, Arizona and elsewhere in the southwestern United States duting the Late Triassic. If the peculiar pattern of damage was caused, as it seems, by a pathogenic fungus then it can be compared in scale with the extensive destruction of Ulmus trees by Dutch Elm Disease [ Ophiostoma (Ceratocystis) ulmi (Buisman)] in recent years in Europe and North America.

The Upper Triassic fern Phlebopteris smithii and its spores

Abstract Phlebopteris smithii (Daugherty) Arnold is a palmate fern leaf that appears to be monopodial, with an odd number (5 to 15) of spreading pinnatifid pinnae. The sori occur in a single row on either side of the pinnule midrib and presumably are not protected by indusia. Each sorus typically contains 14 to 20 sporangia which have oblique annulii. The spores are triangular in polar view with psilate walls and trilete laesurae with straight radii which extend almost to the equatorial margins. They most closely resemble the dispersed spores Diclyophyllidites harrisii which ranges from the Carnian Stage of the Late Triassic to the Bathonian Stage of the Middle Jurassic. Phlebopteris smithii is widely distributed in Upper Triassic to Lower Jurassic units of the United States and is particularly common in the Upper Triassic (middle Carnian) Chinle Formation in the Southwest. Dispersed spores similar to those produced by P. smithii occur also in the same units. P. smithii is one of the oldest representative...

Middle Jurassic strata link Wallowa, Olds Ferry, and Izee terranes in the accreted Blue Mountains island arc, northeastern Oregon

Middle Jurassic strata atop the Wallowa terrane in northeastern Oregon link the Wallowa, Izee, and Olds Ferry terranes as related elements of a single long-lived and complex oceanic feature, the Blue Mountains island arc. Middle Jurassic strata in the Wallowa terrane include a dacitic ash-flow deposit and contain fossil corals and bivalves of North American affinity. Plant fossils in fluvial sandstones support a Jurassic age and indicate a seasonal temperate climate. Corals in a transgressive sequence traditionally overlying the fluvial units are of Bajocian age and are closely related to endemic varieties of the Western Interior embayment. They are unlike Middle Jurassic corals in other Cordilleran terranes; their presence suggests that the Blue Mountains island arc first approached the North American craton at high paleolatitudes in Middle Jurassic time. The authors consider the Bajocian marine strata and underlying fluvial volcaniclastic units to be a basin-margin equivalent of the Izee terrane, a largely Middle Jurassic (Bajocian) succession of basinal volcaniclastic and volcanic rocks known to overlie the Olds Ferry and Baker terranes.

Revision of the biostratigraphy of the Chatham Group (Upper Triassic), Deep River basin, North Carolina, USA

Abstract Paleontological evidence from the Upper Triassic Chatham Group in the three subbasins of the Deep River basin (North Carolina, USA) supports a significant revision of the ages assigned to most of this non-marine continental sedimentary sequence. This study confirms an early(?) or mid-Carnian age in the Sanford subbasin for the base of the Pekin Formation, the lowest unit of the Chatham Group. However, diagnostic late Carnian palynomorphs have been recovered from coals in the lower part of the Cumnock Formation in the Sanford subbasin, and from a sample of the Cumnock Formation equivalent in the Wadesboro subbasin. Plant megafossils and fossil verebrates from rocks in the Sanford subbasin also support a late Carnian age for the Cumnock Formation and its equivalents. The overlying Sanford Formation, which has not yet been dated paleontologically, probably includes beds of Norian age, as over 1000 m of strata may be present between the Cumnock Formation coals (dated here as late Carnian) and the top of the Sanford Formation. This chronostratigraphic interval appears similar to, but slightly longer than, that preserved in the Dan River-Danville and Davie County basins 100 km to the northwest. Our evidence, therefore, indicates that the Chatham Group was deposited over a much longer time interval [early(?) to mid-Carnian through early Norian] than previously was believed.