"Got DZ?—It's Always Better with Petrography"
Timothy Lawton, PhD
S2S Geo, Las Cruces, NM
May24, 2018 9:15-10:00a
Detrital zircon techniques have revolutionized provenance analysis, but commonly are employed at the expense of traditional sandstone petrography. Three parables illustrate the continued utility of, and wisdom of including, classical sandstone petrography in all provenance studies, even in this age of easy access to LA-ICP-MS. Sandstone petrography enhances understanding of at least three major aspects of provenance and stratigraphic studies that employ detrital zircon analysis:
1. Discriminating intra-basinal sediment dispersal. Upper Cretaceous strata of the Cordilleran foreland basin in southern Utah contain different detrital zircon populations that can be attributed to sources in the Sevier fold and thrust belt, basement rocks of the Mogollon highland which lay south of the basin, magmatic arc rocks in California and southern Arizona, or combinations of those three sources. Sandstone composition and texture vary systematically with the detrital zircon suites in the strata. Distributive fluvial fans shed into the foredeep by transverse drainages contain quartzose and quartz-lithic sandstone derived from Jurassic erg deposits and other strata of the thrust belt, whereas axial drainage systems transported lithic and feldspathic sediment from arc and basement rocks that lay south of the basin. Complicated petrofacies patterns and diverse detrital-zircon populations indicate mixing of axial and transverse fluvial sediment. References: Lawton et al. (2003); Lawton et al. (2014a, b).
2. Assessing maximum versus true depositional ages. The ages of young zircons in a sample are commonly interpreted and misinterpreted as representing the depositional age of a stratigraphic unit. In some cases, double dating using U-Pb and (U-Th)/He ages can corroborate maximum depositional ages as actual depositional ages; however, if a sample achieved post-depositional temperatures of ~180° C, this corroboration is not possible. Recognition of neovolcanic grains, including unaltered felsitic grains, sanidine and volcanic quartz, in turbidites of the Upper Jurassic Cucurpe Formation of Sonora demonstrate penecontemporaneous volcanism and deposition. A maximum depositional age of 149 ± 1 Ma calculated from young zircon grains in a sample of volcanic litharenite is therefore likely the depositional age. The Tithonian depositional age is confirmed by ammonites from outcrops elsewhere. Post-depositional burial of as much as 10 km likely has reset the ZHe ages, a prediction yet to be confirmed. Reference: Mauel et al. (2011)
3. Reconstructing continent-scale sediment-routing systems. Permian eolian units in the Late Paleozoic Paradox basin of SE Utah and SW Colorado have long been postulated to have central Pangean (Appalachian) sources, whereas intercalated arkosic fluvial strata are inferred to have local sources in the adjacent Uncompahgre uplift. Upsection stratigraphic changes in the abundance of zircon from Appalachian sources in fluvial and eolian facies corroborate this inference, but stratigraphic variation in QtFL composition, particularly with regard to plagioclase abundance, does not track the change in zircon proportion. Recognition of selective alteration and dissolution of plagioclase in the eolian facies permits reconstruction of former QtFL composition and yields a compositional stratigraphic trend that better matches the detrital zircon content of the sandstones. An important conclusion is that texturally and compositionally mature sandstone in the basin was derived from distant sources not predicted by the local paleogeography of the Ancestral Rocky Mountain province. Reference: Lawton et al. (2015).
Lawton, T.F., Pollock, S.L., and Robinson, R.A.J., 2003, Integrating sandstone petrology and nonmarine sequence stratigraphy: application to the Late Cretaceous fluvial systems of southwestern Utah, U.S.A.: Journal of Sedimentary Research, v. 73, p. 389-406.
Lawton, T.F., Eaton, J.G., Godfrey, K.N., and Schellenbach, W.L., 2014a, Compositional, paleontological and detrital-zircon data from Cretaceous strata of the Henry Mountains basin and implications for connections with dispersal systems of Wahweap and Kaiparowits formations in southern Utah, U.S.A., in MacLean, J. S., Biek, R. F., and Huntoon, J. E., eds., Geology of Utah's Far South: Salt Lake City, Utah, Utah Geological Association Publication 43 (compact disc), p. 373-395.
Lawton, T.F., Schellenbach, W.L., and Nugent, A.E., 2014b, Late Cretaceous fluvial-megafan and axial-river systems in the southern Cordilleran foreland basin: Drip Tank Member of Straight Cliffs Formation and adjacent strata, southern Utah, USA.: Journal of Sedimentary Research, v. 84, p. 407-434, doi: http://dx.doi.org/410.2110/jsr.2014.2133.
Lawton, T.F., Buller, C.D., and Parr, T.R., 2015, Provenance of a Permian erg on the western margin of Pangea: Depositional system of the Kungurian (late Leonardian) Castle Valley and White Rim sandstones and subjacent Cutler Group, Paradox Basin, Utah, USA: Geosphere, v. 11, p. 1475–1506, doi:1410.1130/GES01174.01171.
Mauel, D.J., Lawton, T.F., González-León, C.M., Iriondo, A., and Amato, J.M., 2011, Stratigraphy and age of Upper Jurassic strata in north-central Sonora, Mexico: Southwestern Laurentian record of crustal extension and tectonic transition: Geosphere, v. 7, p. 495-512; doi: 410.2110/jsr.2011.2145.