Western Equatorial Pangea
Andrew B. Heckert, Co-Leader, Dept. of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM
Roberto Molina-Garza, Co-Leader, Dept. of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM
Upper Triassic to Lower Jurassic rocks in the American Southwest represent a vast (up to 2.5 million km2) depositional basin on the western interior and margin of Pangea, approximately 5-10 degrees north of the equator. A focused coring program in these rocks would afford an excellent opportunity to conduct high-resolution studies testing hypotheses of Triassic-Jurassic chronology, paleogeography, paleoclimate, biotic evolution, and basin development. Classic exposures of the Chinle and lower Glen Canyon groups in the American southwest span the Triassic-Jurassic boundary and contain abundant megafossil plants, palynomorphs, invertebrates, microfossils, and nonmarine vertebrates. These nonmarine deposits represent a diverse suite of depositional environments and ecological niches of Mesozoic Pangea, some of which are, and many of which are not represented in other Pangea basins.
A focused coring program in Triassic and Jurassic strata of SW North America would lend insight into issues of Pangean chronology, paleogeography, paleoclimate, and biotic evolution across the Triassic-Jurassic boundary by accomplishing the following:
(1) developing a high-resolution magnetostratigraphy of Chinle and lower Glen Canyon strata to facilitate correlation with the chronology established by the Newark coring project.
(2) refining the paleogeography, particularly the paleolatitude, of equatorial western Pangea during Late Triassic-Early Jurassic time.
(3) examining how paleoclimates are expressed through time in the sedimentary record of western Pangea during this key interval, and teseting this paleoclimate record against that of the equilatitudinal Newark Supergroup.
(4) testing current hypotheses of lithostratigraphic and biostratigraphic correlation and the development of regional unconformities within the Chinle-Glen Canyon interval and their possible relationship to Triassic-Jurassic sea-level.
We envision a drilling program that, at a minimum, would sample the Late Triassic-Early Jurassic interval through shallow (< 2 km) cores from at least two locations. One core would be collected on the Colorado Plateau, and a second on the Great Plains. The Colorado Plateau core would penetrate the Early Jurassic Kayenta Formation and Wingate Sandstone, as well as the entire Late Triassic Chinle Group. The High Plains core would penetrate a similar stratigraphic interval of the Late Triassic, including the latest Triassic lacustrine deposits of the Redonda Formation.
Within the framework provided by current biostratigraphic and lithostratigraphic data and a magnetostratigraphy linked to the Newark Supergroup polarity sequence, these cores would also be used refine the existing regional correlation and nomenclature of these rocks. We will utilize this and other lithological data to test existing models of non-zonal (megamonsoonal) climate. Additionally, some Chinle deposits appear cyclical, and an improved chronology would test the possibility that these cycles are a result of orbital forcing and/or event stratigraphy.
STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENTS
The Upper Triassic Chinle Group encompasses strata assigned to about 50 lithostratigraphic units in the western United States ranging from West Texas to Nevada and Idaho to New Mexico. Most of these strata were deposited in a single depositional basin associated with a large fluvial system with paleoflow to the west and a drainage basin comparable to the Congo Basin of equatorial Africa. The depositional environments include channels, overbank/crevasse splays, floodplains, and minor lacustrine facies, with increasing evidence of eolian and lacustrine deposition at the top of the section. Three unconformity-bound sequences have been observed in these deposits. The first two sequences consist of basal channel-fill conglomerates and sandstones that fine upwards into dominantly flood-plain mudstones and paleosols. The third sequence consists primarily of interbedded siltstones and fine sandstones, and demonstrates an increasing eolian trend. Existing biostratigraphic correlations in the Chinle Group allow the discrimination of at least four stratigraphically superposed Upper Triassic vertebrate faunas that span from Carnian to late-Norian (Rhaetian?) age. This is the best-studied succession of Late Triassic nonmarine vertebrate faunas in the world, and can be used to correlate Chinle strata to Triassic basins in eastern North America, Greenland, Europe, North and South Africa, South America, and India.
The uppermost Triassic and Lower Jurassic Glen Canyon Group includes the Wingate Sandstone, Moenave Formation, Kayenta Formation, and Navajo Sandstone. Most relevant for this discussion is the lower Glen Canyon Group, principally the Kayenta Formation and the underlying Wingate Sandstone. The Wingate is a primarily eolian unit which, based on vertebrate biostratigraphy and preliminary magnetostratigraphy, encompasses the Triassic-Jurassic boundary, whereas the overlying Kayenta Formation is dominated by fluvial deposits and contains a rich vertebrate fauna of Early Jurassic (Sinemurian) age similar to those known from China, South Africa, South America, and Europe. The Moenave Formation is homotaxial to, and intertongues with, the Wingate Sandstone. Hettangian palynomorphs have been reported for parts of the Moenave Formation.
The paleomagnetism of the Triassic-Jurassic interval in the American southwest has been extensively studied for over 40 years. Hematite cemented sandstones and siltstones of the Chinle and lower Glen Canyon groups are typified by well-defined characteristic magnetizations, which have been fundamental in the definition of the North American apparent polar wander path. Magnetostratigraphy of the Chinle Group has also been studied extensively, and has been used to corroborate regional-scale litho- and biostratigraphic correlations. The magnetostratigraphy of the Glen Canyon Group is currently limited to the Kayenta Formation. The nature of Chinle and Glen Canyon outcrops and depositional systems are such, however, that high-resolution studies have not yet been possible, due to difficulties associated with sampling fine-grained, easily eroded intervals.
Vertebrate fossil assemblages in the Chinle Group are the best biostratigraphic tool for regional and intercontinental correlation of the Upper Triassic across Pangea. Unfortunately, the temporal resolution required for detailed, high-resolution study of specific questions regarding the tempo and style of regional and global climate modification, evolutionary trends, and paleogeography can only be afforded with magnetostratigraphy at this time. The possibility for a direct correlation of the magnetostratigraphy of the Chinle Group with that established by the Newark Supergroup coring project offers the best opportunity to link the rich nonmarine fossil and interpreted climate records of the American southwest with the Late Triassic and Early Jurassic time scales.
BASIN DEVELOPMENT AND EVOLUTION
The Chinle and younger strata in the American Southwest represent a fundamentally different setting than the Newark Supergroup rift basins, but (1) cover the same time interval, and (2) approximate the same paleolatitude, and therefore play directly into issues of climate, chronology, and paleobiology. Specific questions to address include: How are Tr-J climates in the western equatorial Pangea margin and interior similar to and/or different from those of the Newark Supergroup? Are biochronological schemes of the Chinle Group consistent with those of other Pangean basins and the marine record?
The sedimentary record of any basin necessarily reflects the complex interplay of tectonics, climate, and base-level change. The Newark Supergroup record allows for detailed chronology of climate-dominated basin fill. Forcing mechanisms in the Chinle (including the Dockum Group) are less clear, but are probably more likely tied to eustatic, and, locally, tectonic regimes. Notably, persistent inter- and intra-group Triassic and Early Jurassic regional unconformities in the American Southwest appear to coincide with intervals of lowered global sea level. The program outlined here would better constrain the timing and duration of these unconformities and test the hypothesis that they are in some way tied to global sea-level change.
We propose two targets, one on the Colorado Plateau and another on the southern High Plains. The Colorado Plateau candidate is associated with the Comb Ridge monocline, and the High Plains candidate would be in the Tucumcari Basin in eastern New Mexico. The Comb Ridge and Tucumcari Basin targets are the best possible for studying the Norian-Jurassic interval in the American Southwest. Sound lithostratigraphic and biostratigraphic correlation of the two principal target sections to additional localities would greatly facilitate correlation of Carnian faunas into the Newark Supergroup magnetostratigraphy.
Because a great deal of the questions we want to answer have their roots in the Carnian, other possible targets include the very well-constrained Carnian-Norian sections in the immediate vicinity of the Petrified Forest National Park (Arizona) and Post, Texas.
Null hypothesis: A zonal climate model would predict that the Chinle climates should be equivalent to at least some part of the southern Newark Supergroup basins, as both are at the same latitude. The alternate hypothesis is that Pangean climate is non-zonal, and that a different (monsoonal?) climate for the Chinle is an expression of this non-zonal climate pattern.
Of particular interest is a drying trend observed in the latest Triassic and early Jurassic of the Colorado Plateau. Specifically, eolian sandsheets in the Rock Point Formation become more prevalent upsection until reaching the Wingate Sandstone, which is an almost entirely eolian unit. Magnetostratigraphic information obtained from cores would help determine whether this is a result of true climate change or simply an artifact of the northward migration of the Chinle basin into a zone of more prevalent aridity.
The Chinle-Kayenta section we envision drilling is approximately 1 km thick, or nearly an order of magnitude thinner than that of the Newark Supergroup. To date, it is not entirely clear whether the Chinle section is less complete, or just condensed. Obviously, the magnetostratigraphy can answer this question.
To date, significant debate revolves around the question of the nature and extent of Laramide deformation of the Colorado Plateau, specifically whether there was signifcant clockwise rotation of the Colorado Plateau during Laramide time. Because we plan to test localities both on the CP and the stable craton, we will be able to present evidence that bears on this question. Superficially, this question lies beyond the scope of a Triassic-Jurassic boundary problem, but the answer to this question bears directly on the Tr-J interval by affecting how paleolatitudes are interpreted.
The Newark Supergroup boasts a series of astronomically-calibrated cycles that appear to be largely dependent on climatic forcing. To date, few Tr-J deposits in the American Southwest appear to contain any such cycles.
1-1.5 km cored well in southeast Utah, probably near the Comb Ridge monocline
1 km well in the Tucumcari Basin
All holes must be logged
The Tucumcari Basin well is challenging because the strata there are essentially flat-lying, and thus there is some question regarding orientation of the core. However, the Comb Ridge site would penetrate steeply dipping (up to 45 degrees) strata.