Big rivers are the bloodlines of continents. Transcontinental drainage networks carry a detrital record of active and ancient orogenic events whose basement roots and derived sediments remain exposed. Provenance analysis of detrital zircons—and other resistant minerals—is the primary method used to reconstruct and interpret fluvial systems and continental tectonics through time. In this issue of Geology, the Cretaceous–Paleocene evolution of big-river networks draining North America was inferred from detrital zircon U-Pb geochronology (Blum and Pecha, 2014, p. 607), a study reflecting an important confluence of user-friendly analytical technology with a large, critical mass of data on ages of basement and detrital rocks.
Understanding continent-scale fluvial systems and the voluminous, resource-rich sediment accumulations at their termini has long engaged the imagination of sedimentologists and sedimentary petrologists (Potter, 1978; Dickinson, 1988; Galloway et al., 2011). Recognition of large, ancient fluvial systems, however, can be difficult due to difficulties in facies interpretation (Miall, 2006) and scarcity of preserved trunk-river deposits on cratons.
The promise of U-Pb detrital-zircon geochronology for reconstruction of continent-scale drainage systems became apparent as soon as ages of individual sandstone grains could be determined. Early efforts using small numbers of grains produced spectacular results in revealing sediment-dispersal systems: Neoproterozoic paleorivers flowing 3000 km across Rodinia (Rainbird et al., 1997); a Devonian fluvial system transporting detritus from the Caledonide belt in east Greenland to Arctic Canada (McNicoll et al., 1995); and Upper Triassic sand delivered to northwestern Nevada (USA) from Oklahoma’s Amarillo-Wichita uplift or further (Riggs et al., 1996).
Much research using the detrital zircon U-Pb technique has focused on fluvial lineages, i.e., the correlation, connection, and provenance of drainage systems, with some caveats: (1) specific original sediment-source areas cannot always be identified; (2) correct contemporary tectonic setting or uplift timing cannot always be recognized from detrital grain assemblages; and (3) equivalent fluvial systems cannot always be rigorously correlated.
(1) Processes inherent to the sedimentary system or basement-rock distribution can prevent unequivocal identification of sediment-source area. Recycling of resistant zircon grains can cause derivative sedimentary rocks to constitute more important grain sources than primary crystalline rocks. For instance, Permian and Jurassic eolianites in the Sevier orogenic belt contain zircons with age populations of 1.3–1.0 Ga, 750–500 Ma, and 500–320 Ma (Dickinson and Gehrels, 2003). Those ages are uncommon in United States Cordilleran basement rocks, but abundant in detritus from the Cordilleran thrust belt, and thus can be used to distinguish synorogenic fluvial systems draining eolian strata in the thrust belt (Leier and Gehrels, 2011). In addition, the yield of zircon grains per volume of igneous rock (“zircon fertility”) varies with tectonomagmatic setting (Moecher and Samson, 2006); thus, some grain ages, e.g., 1.2–1.0 Ga Grenville ages from granitoids and their derivative sandstones, are abundant far from their original basement rocks (Rainbird et al., 1997; Gehrels et al., 2011; Laskowski et al., 2013). The absence or rarity of such widely occurring grain ages therefore might indicate a local primary basement source. Finally, not all primary basement rock ages permit unequivocal determination of sediment sources. The Sveconorwegian orogen of Baltica and the southwestern part of the Amazonian craton both contain basement with distinctive ages of ca. 1500 Ma (Bingen and Solli, 2009; Cardona et al., 2010), and either source might have provided Mesoproterozoic grains to rivers flowing to western Pangea (Lawton and Parr, 2013; Soreghan and Soreghan, 2013).
(2) Zircons may be rare or absent in a sample or deposit. Zircon availability during the lifetime of a sediment-transport system requires exposure of basement or derivative sedimentary rock sources (Thomas, 2011). In addition, the rate of orogenic unroofing strongly controls zircon populations, a vexing problem when synorogenic igneous zircons might not be exhumed at all, or only long after uplift, such as in collisional orogens and broad thermal uplifts preceding rifting. For example, the Alleghenian orogenic event (ca. 330–270 Ma) is not recorded in the Appalachian foreland basin because basement rocks with zircons of that age were not yet exposed (Moecher and Samson, 2006; Thomas, 2011). Zircons of that age range do occur in Lower–Middle Jurassic eolian strata of the Colorado Plateau, transported there from coeval uplift of the Atlantic rift flank (Dickinson and Gehrels, 2003; Rahl et al., 2003).
(3) Equivalence of grain-age populations in compared samples is critical to determine river lineages. However, varying proportions of grain ages can result from incomplete mixing of river bedload, as has been argued for closely spaced samples collected between tributary inputs along the Solimões River (Amazon, Brazil) (Mapes, 2009). In addition, rigorous statistical comparisons of samples require improvement. The Kolmogorov-Smirnov test is currently favored in statistical sample comparisons, but is strongly sensitive to sample size (Vermeesch, 2013) and tends to reject equivalence of sample pairs if age populations contain different numbers of grains (Gehrels, 2012).
The difficulties of source-area discrimination (factor 1) may be solved through multiproxy analytical techniques; e.g., Lu-Hf analysis of dated zircon grains, which may refine criteria to discriminate source areas of similar age. With development of efficient multiproxy analytical techniques, more extensive fluvial networks will be delineated, including “super” transcontinental rivers across current continental margins. Existing reconstructions of this type include Triassic Pangean sediment dispersal from Siberia to northern Alaska (Miller et al., 2013), and a fluvial connection between enormous volumes of Upper Triassic sandstone under western Mexico and headwaters in Amazonia (Ortega-Flores et al., 2014).
Combining geochronology and thermochronology on single detrital zircon grains (“double dating”; Carrapa, 2010), including (U-Th)/He analysis, with low-temperature thermochronometric analyses of associated grains, such as apatite fission-track (AFT) and apatite (U-Th)/He (AHe) methods, will provide critical insight into headwater uplift ages and lag times between crystallization and exhumation (factor 2), and provide additional criteria to define potential source regions. With river lineages well established, the focus on source and sink may well shift to questions about the genesis of headwater topography. Some, but not all, modern river systems record their headwaters’ tectonic setting. The trunk river of the Amazon network, for instance, contains ∼30% Andean-derived zircon grains, with the younger ones providing evidence for an active arc in its headwaters (Mapes, 2009). In contrast, U-Pb grain ages in the Mississippi watershed represent the cumulative record of the continent’s tectonic history (Craddock and Kylander-Clark, 2013), but without low-temperature detrital thermochronometric analyses, including zircon (U-Th)/He, AFT and AHe analyses, it would be impossible to conclude that its eastern part drains an ancient orogen undergoing persistent long-term uplift (McKeon et al., 2014), whereas its massive western part transports detritus from the dynamically elevated greater Rocky Mountains (Schmandt and Humphreys, 2010). Low-temperature detrital thermochronology thus can assist in better understanding exhumation lag times and more closely matching transcontinental rivers with the tectonic reasons for their topography.
To conclude, our perception of what constitutes a big river network will grow in scale, partly because our view of transcontinental rivers is biased by the present continents, which are small compared to those of other times in Earth history. The study of Blum and Pecha (2014) convincingly demonstrates, by means of detrital-zircon U-Pb analysis, how even the most extensive post-Pangean drainage systems have evolved on a single continent. The lineages and genesis of big river systems will continue to provide stimulating investigative challenges for the foreseeable future.
I thank Peter Copeland, Andrew Leier, Luigi Solari, and Ellen Thomas for comments.
