There and Back Again: Recycling of the Appalachian Signature in DZ U-Pb Records of Phanerozoic North America

Appalachian-derived detrital zircon (DZ) grains with U-Pb ages from ca. 1300–275 Ma accumulated along the western Laurentian margin during the late Paleozoic and early Mesozoic [1-4]. During the late Jurassic, following development of the Mesozoic Western Cordillera (hereafter, MWC), this “Appalachian signature” (sensu Thomas et al. [5]) was recycled and transported back to the east, where it became an important component in the DZ U-Pb geochronological record of the U.S. and Canadian Western Interior foreland basin [6-9] and the Gulf of Mexico (GoM) passive margin [10]. Recycling of DZ grains is becoming increasingly well-known [11], especially on a local to regional scale, but the broader scales of zircon and associated sediment recycling remain less well-known. Here, we examine DZ U-Pb age distributions from Pennsylvanian to modern stratigraphic units across North America to illustrate the transcontinental scale, trajectory, and stratigraphic significance of the recycled Appalachian signature (see Methods in the Supplementary Material 1). We argue that recycling of the Appalachian signature reflects major continental- to regional-scale tectonic and geodynamic events that drive continental-scale drainage reorganizations and that recycling of the Appalachian DZ U-Pb signature and its associated Appalachian-derived sediments dominates the record of sediment dispersal to Mesozoic and Cenozoic basins across North America.

The western Laurentian passive margin of southern North America was an active depocenter from the Neoproterozoic through the Early Jurassic [12]. DZ U-Pb data from the Grand Canyon in Arizona [4], the Bighorn Basin in Wyoming [1], and elsewhere show that pre-Mississippian clastics were generally derived from the Paleoproterozoic Central Plains orogens (Yavapai-Mazatzal and Midcontinent granite-rhyolite provinces, ca. 1800–1600 Ma and 1550–1300 Ma, respectively) of the midcontinental and southwestern United States (Figure 1) and the Mesoproterozoic Grenville orogen in eastern Canada and the United States [13]. Grenville zircon grains first reached western Laurentia during the Neoproterozoic and had been dispersed from the present-day Arctic to the southwestern United States by the Early Paleozoic [14-19].

The Appalachian orogenic system extends >5000 km from eastern Canada to West Texas and northern Mexico [20, 21] and formed after Neoproterozoic rifting of the post-Grenville Iapetus Ocean. Arc magmatism was common in the Ordovician Taconic (ca. 480–440 Ma) and Devonian to Mississippian Acadian phases (ca. 400–350 Ma), whereas during the Mississippian through early Permian (ca. 320–275 Ma), the Alleghenian and Ouachita-Marathon collisional belts formed on Laurentia during the assembly of Pangea, with a magmatic arc on Gondwana. The provincial “Appalachian” DZ U-Pb signature initially accumulated in clastic wedges of the Appalachian foredeep [5], the Appalachian backbulge [22], and the deepwater Ouachita trough [23] and represents the integrated record of sediment sources within the Appalachian orogenic system (sensu Lawton et al. [2]). This signature is dominated by what we refer to as the Appalachian-peri-Gondwanan-Grenville (APG) age group, which is composed of (a) Paleozoic DZ U-Pb ages that reflect the different phases of Appalachian orogenesis (10%–15% of the total), (b) small (<5%) contributions from Neoproterozoic to Cambrian peri-Gondwanan terranes (ca. 850–510 Ma) that accreted to, or formed in, eastern and southern Laurentia prior to and during the assembly of Pangea, and (c) Mesoproterozoic Grenville age groups that reflect the erosion of Grenville inliers within the Appalachians (40%–60%) [5, 24].

Isotopic and DZ U-Pb data show that Appalachian-derived detritus reached western Laurentia by ca. 450 Ma [1-25–27,25-27]. In the initial stages of the journey west, the provincial Appalachian signature became increasingly cosmopolitan. For example, during the Carboniferous, the provincial Appalachian signature was diluted by contributions from the Paleoproterozoic Central Plains orogens that likely represent the recycling of Paleozoic sedimentary rocks of the northern U.S. midcontinent [22, 23]. This diluted and more cosmopolitan signal accumulated in the backbulge and peripheral parts of the Appalachian foreland-basin system, as well as the synorogenic Ouachita deepwater trough [23]. The latter is especially important because collision during the Latest Pennsylvanian and Permian assembly of Pangea also resulted in the transformation of the Ouachita deepwater sink into the Ouachita fold-and-thrust-belt source.

While it is clear the Appalachian signature was transported to western Laurentia during the Carboniferous, the sediment-routing systems that transferred this signal remain poorly defined through the Permian [28]. Chapman and Laskowski [29] suggested that longshore drift along the southern Laurentian margin played a significant role. However, Allred and Blum [23] consider it unlikely that sediment was routed directly to the west from the central and southern Appalachians since much of the eastern United States drained to the Ouachita deep-water trough during sea-level lowstands, much of the U.S. midcontinent was a flooded shelf dominated by carbonates during sea-level highstands, and the ancestral Rocky Mountains represented a significant barrier to westward transport. An alternative view by Leary et al. [3] suggests that part of the APG age group that populates the western Laurentian margin may have been derived from the northern Appalachians and perhaps as far north as the Pangean Ellesmerian and Caledonian orogenies of present-day Arctic Canada and parts of Greenland [30, 31] (Figure 1). Leary et al. [3] further argue this long-distance transfer may have occurred via transcontinental fluvial systems that have not been identified, and/or via north-to-south longshore drift along the western margin.

The breakup of Pangea caused the opening of a new rift basin that would evolve to become the GoM passive margin basin [32]. Frederick et al. [33] show the Late Triassic synrift Eagle Mills Formation throughout the southeastern United States was deposited within an internally drained basin, with the provincial Appalachian DZ U-Pb signature dominating the northern and western extent of the rift system (Figures 2 and 3). Farther west, however, DZ U-Pb data from the Late Triassic Chinle Formation represent a transcontinental fluvial system that routed the Appalachian signature from the Ouachita-Marathon collisional belt to the western Laurentian margin [13, 26, 34-36] (see also Supplementary Material 2). Along the way, the primary Appalachian signature was further diluted by the Paleoproterozoic Central Plains orogens age groups, first from recycling sedimentary rocks of the Ouachita-Marathon collision belt, and then directly from Paleoproterozoic rocks in the Mogollon highlands of central Arizona. The Chinle also has a significant ca. 275–200 Ma age group, which is the primary fingerprint of contributions from the early MWC magmatic arc [26].

The provincial Appalachian signature characterizes DZ U-Pb ages for modern river systems that discharge to the eastern GoM and has changed little since the Triassic [10, 37]. However, in the western United States, the development of the MWC resulted in the uplift and incorporation of western Laurentian passive-margin strata into the eastward-propagating Sevier fold and thrust belt [1, 38, 39] and the formation of the associated retroarc Sevier foreland-basin system [40]. By the middle Jurassic, river systems in the west were flowing generally eastward; this large-scale reversal of flow was the first stage in the continental-scale reorganization of North American sediment routing [41] and in the transport of a recycled and more cosmopolitan Appalachian signature back to the east. This recycled signature included additional contributions from the Paleoproterozoic Central Plains orogens exposed in the southwestern United States [8], as well as volcanogenic zircon grains from the MWC magmatic arc, which collectively represent the fingerprints that define this change in source and transport direction.

Through the Early to mid-Cretaceous, this recycled Appalachian signature was sequestered within Sevier foreland-basin strata [6, 9] and/or transported farther east where it merged within the U.S. midcontinent with east-derived sediment dominated by the primary Appalachian signature; this mixed recycled and primary Appalachian signature was then routed north to become the dominant DZ U-Pb signature in the Early Cretaceous foreland-basin phase of the Western Canadian Sedimentary Basin, with the likely sink being the Boreal Sea [7, 41]. The zenith of this sediment-routing system is represented by the continental-scale river system that deposited the Aptian McMurray Formation and younger members of the Early to mid-Cretaceous Mannville clastic wedge, which host the Alberta Oil Sands [7]. By the Late Albian and Cenomanian, the Western Interior Seaway had expanded to the north and south, connecting the Boreal Sea to the GoM, which disrupted the convergence of west- and east-derived tributary systems. Cenomanian fluvial systems that deposited the Dakota group, as exposed today in the Rocky Mountains of Wyoming, Colorado, and New Mexico, then delivered the cosmopolitan recycled APG age group to the western shoreline of the seaway, and the provincial primary APG age group to the eastern shoreline, which extended from South Dakota to Texas (Figures 2 and 3) [42-44]. However, prior to the latest Cretaceous, the GoM drainage area remained relatively small and restricted to the southeastern United States, south of the Appalachian and Ouachita collision belts [8, 43]. East of the Mississippi embayment, the GoM DZ U-Pb record is dominated by the provincial primary Appalachian signature, whereas to the west, the GoM record is slightly more cosmopolitan because sediment was derived from erosion of Pennsylvanian and Permian clastic rocks within the Ouachita fold and thrust belt [10].

Beginning in the Late Cretaceous, the Sevier foreland basin was undergoing Laramide deformation, but Late Cretaceous to Early Paleocene fluvial systems with headwaters in the MWC magmatic arc and the Sevier fold and thrust belt continued to flow east into and through evolving Laramide topography to the western shore of the Western Interior Seaway. Withdrawal of the seaway initiated the second stage in the continental-scale reorganization of North America sediment routing: rivers with headwaters in the MWC flowed east and then south, directly to the western GoM in Texas, or continued eastward to join an ancestral Mississippi River and its Appalachian-derived tributaries within the Mississippi embayment [10]. DZ U-Pb data from the Paleocene-Eocene Wilcox Group, which extends across the GoM coastal plain from Texas to Alabama, show three dominant sediment-routing systems. The western GoM was fed by a large fluvial system that drained the southwestern United States: the cosmopolitan DZ U-Pb age distribution is dominated by the Western Cordillera magmatic arc and Central Plains orogens but includes a small recycled APG age group. Farther east within the Mississippi embayment, a west- and east-derived tributary network converged to form an ancestral Mississippi River. West-derived tributaries had headwaters that extended into and beyond the central and northern Rocky Mountains to British Columbia [45] and delivered sediments with a cosmopolitan-recycled APG age group to the Mississippi embayment and central GoM, where it accounts for ~58% of the total DZ U-Pb ages. East-derived tributaries like the paleo-Tennessee River continued to have headwaters in the Appalachians and delivered sediment with the provincial Appalachian signature and primary APG age group to the Mississippi embayment (~82%; Figure 3), whereas farther east in Mississippi and Alabama, relatively small Wilcox rivers derived sediment from the southern Appalachians and delivered a provincial Appalachian signature and the primary APG age group directly to the eastern GoM [10, 41].

Several papers have suggested that the Saglek Basin of the Canadian Atlantic margin was a major depocenter for a transcontinental fluvial system with headwaters in the Canadian Rockies that has historically been referred to as the Bell River [41, 46-48]. Earlier work was not well supported by empirical data, but new results presented by Sears and Beranek [49, 50] and Corradino et al. [50] show the recycled APG age group, including magmatic zircon grains with Mesozoic ages, was delivered to the Saglek Basin coeval with Paleocene Wilcox deposition in the GoM and continued to accumulate along the Canadian Atlantic margin through the Miocene. Farther south and west, however, the headwaters of Paleocene Wilcox fluvial systems that delivered sediment from the MWC to the western and central GoM were tectonically dismembered by the Oligocene, and the western extent of GoM drainage was thereafter restricted to the central and southern Rockies in Colorado, New Mexico, and northern Mexico [10]. Nevertheless, DZ U-Pb records from Oligocene and Miocene strata of the GoM coastal plain in Texas, Louisiana, and Mississippi show the pattern of convergence of west- versus east-derived river systems within lowlands of the Mississippi embayment [10, 51].

Younger samples of Plio-Pleistocene age from the Mississippi embayment also reflect this convergence (see Supplementary Materials 3 and 4). DZ U-Pb signatures from the eastern valley margin in Illinois and Kentucky continue to be very provincial, dominated by the primary APG group, which comprises 78% of the total; like earlier east-derived DZ U-Pb age distributions, contributions from the Central Plains orogens comprise <10% of the total, and there is no contribution from the MWC magmatic arc. By contrast, samples from the western valley margins of the lower Mississippi in Missouri and Arkansas show a cosmopolitan DZ U-Pb signature, where the recycled APG group comprises ~60% of the total, the Central Plains orogens comprise 20%–25%, and the Western Cordillera magmatic arc comprises ~15%–18%.

The modern Lower Mississippi River, the overwhelmingly dominant conduit for sediment transfer to the GoM passive margin and deepwater basin, continues to show a mixing of western and eastern signatures [10]. DZ U-Pb data from the Rocky-Mountain-sourced Missouri and Arkansas rivers, which provide most of the sediment load for the lower Mississippi River [52], include the recycled APG age group (~50%), as well as significant contributions from the Central Plains orogens (~10%–15%) and Western Cordillera magmatic arc (~9%) age groups. Contributions from the Central Plains orogens age group likely include detritus directly derived from erosion of basement rocks exposed in the Laramide Rockies, but modern Missouri River sediment supply is known to be dominated by erosion and recycling of Sevier foreland-basin strata [52], such that each of the contributing DZ U-Pb age groups in the Missouri system likely includes a dominant recycled component that was originally eroded from the Sevier fold and thrust belt, deposited in Mesozoic sedimentary rocks of the Sevier foreland basin, then eroded again and transported within the late Cenozoic to modern Missouri-Mississippi system. By contrast, DZ U-Pb data from the modern Ohio River system, including the Tennessee River, continue to be exclusively dominated by the provincial Appalachian signature, indicating sediments continue to be derived directly from the Appalachians or by recycling of Appalachian foreland-basin strata (Figures 3 and 4).

The provincial APG DZ U-Pb signature reflects the integrated contributions of sediment from the Appalachian orogenic system (Figure 3) and dominates the provenance record of the Late Paleozoic Appalachian foreland-basin system and the associated Ouachita deepwater trough. Today, some 350 Myrs later, 70%–85% of the DZ U-Pb ages in rivers that drain the Appalachians still belong to this very provincial primary APG age group.

The primary Appalachian DZ U-Pb signature was dispersed across North America during the late Paleozoic through the middle Mesozoic where it accumulated in and became an important component of the Western Laurentian passive margin succession. However, by the middle-late Jurassic, ca. ~160 Ma, development of the MWC resulted in the uplift of Western Laurentian margin strata, incorporation of sandstones containing the Appalachian signature into the eastward-migrating Sevier fold and thrust belt, and initiation of eastward-flowing river systems. The APG age group was then eroded, recycled, and diluted with contributions from the Paleoproterozoic Central Plains orogens and the Western Cordillera magmatic arc, and a very cosmopolitan recycled APG age group was then transported back to the east by river systems within the Sevier foreland-basin system. Today, the Mississippi River delivers a mixture of recycled and primary APG age groups to the GoM. This mixture includes zircon grains that were initially produced during the Nuna, Rodinia, and Pangea supercontinent cycles, as well as the most recent orogenic system in western North America.

The journey back to the east therefore resulted in the Appalachian DZ U-Pb signature, and presumably associated Appalachian-derived sediment as well, comprising important constituents in major Mesozoic and Cenozoic North American depocenters, including the middle Jurassic through Late Cretaceous Sevier foreland basin, the Early Cretaceous foreland-basin phase of the Western Canadian Sedimentary Basin, the Cenozoic Canadian Atlantic passive margin, and the Late Cretaceous through Cenozoic U.S. GoM passive margin and deepwater basin. We interpret the eastward propagation and return of the recycled APG group to have taken place in a stepwise manner that reflects the following major tectonic and geodynamic changes:

1. During the middle to late Jurassic through Late Cretaceous, the recycled APG group was dispersed by east-flowing rivers that flowed generally transverse across the Sevier foreland basin in the U.S. west-derived Late Jurassic Morrison, and Early to mid-Cretaceous Dakota Group sandstones are dominated by the recycled and cosmopolitan APG age group (Figure 3). This recycled APG group includes a relatively high percentage of peri-Gondwanan-ages (Figure 4(a); ca. 800–500 Ma). Moreover, for the primary APG age group, Appalachian DZ U-Pb ages contribute 10%–20% of the total APG age distribution, with Grenville contributions of 75%–85%. The recycled APG age group, by contrast, displays a relative increase in Appalachian contributions (contributions up to 25%–30%) and decreases in Grenville U-Pb ages (contributions of ~50%–75%).

2. During the Early Cretaceous, the foreland-basin system was not generally flooded by the Western Interior Seaway. Hence, rivers transporting the recycled APG group, derived from the west, converged with Appalachian-sourced rivers transporting the primary APG group from the east and routed sediments to the eastern margins of the Western Canadian Sedimentary Basin to produce the Mannville clastic wedge. At this time, the foreland-basin system coincided with an eastward-migrating axis of dynamic subsidence that steered this continental-scale river system to the north, toward the Boreal Sea [53, 54].

3. The Western Interior Seaway was partially to completely flooded from the Albian through the Late Cretaceous, forming two separate continental-scale landmasses that have been referred to as Appalachia to the east and Laramidia to the west [55, 56]. During times when the seaway was at its greatest extent, the primary APG was transported to the west and sequestered along the western and southern shorelines of Appalachia, whereas the recycled APG was transported to the east and sequestered along the eastern shoreline of Laramidia (Figures 2,³-4).

4. With the withdrawal of the seaway in the latest Cretaceous and early Paleocene and proposed mantle-driven dynamic topography, major river systems were steered to the Canadian Atlantic margin from the northern half of North America and to the GoM passive margin from the southern half [53, 54].

Late Cretaceous to Paleocene drainage reorganization produced the modern template for sediment routing to the GoM [41]. There has been no routing of Appalachian-sourced sediment to the west since the Jurassic, but the recycled Appalachian signal from the west is found in the Cenozoic Atlantic margin to the north and the GoM to the south and is generally more significant than the DZ U-Pb signature of the Western Cordillera magmatic arc per se. We suggest the longevity, magnitude, and significance of the Appalachian signature throughout the Phanerozoic represent the concatenation of two provincial terranes from two separate supercontinent cycles and argue that the primary and recycled Appalachian-Grenville DZ U-Pb age groups are the most recognizable components of North American sediment routing throughout the Phanerozoic. The Appalachian orogenic system still provides the primary APG signal, and its associated sediments, to the eastern GoM and the lower Mississippi Valley, but this remains a small component of the total Mississippi River DZ U-Pb signal. By contrast, the recycled APG dominates the DZ U-Pb signal from the west and dominates the lower Mississippi Valley. Collectively, there is a larger level of significance as well: to the extent that DZ grains are transported with, and serve as a proxy for, the quartz-rich sediment load as a whole, the recycled Appalachian signature and its associated quartz-rich sediment load have been to the western Laurentian margin and back and dominate the GoM passive margin basin to this day.

We thank the Indiana Geological and Water Survey (IGWS) for support during manuscript preparation. We thank Matt Johnson (IGWS) for assistance with ArcPro, and Sarah Burgess and Casey Jones for guidance on figure art. We thank the staff of the Arizona LaserChron Center for assistance with DZ U-Pb analyses. The manuscript benefitted from insightful reviews from Andrew Laskowski and an anonymous reviewer. This research was primarily funded by the Ritchie Endowment and the Scott and Carol Ritchie Distinguished Professorship at the University of Kansas.

The authors declare that there is no conflict of interest regarding the publication of this article.

All detrital zircon U-Pb ages in this study are provided in the supplemental files or are available from previous publications by the authors (i.e., Blum and Pecha [41], Blum et al. [10], Frederick et al. [33], and Allred and Blum [23]); U-Pb ages from additional Pennsylvanian samples are also available from Gehrels et al. [4] and Thomas et al. [5].