The Neihart Quartzite and LaHood Formation are the lowermost units exposed in the Helena embayment, which forms the eastern and southeastern margins of the Belt Basin. Ages of detrital zircons from the Neihart Quartzite (quartz arenite) and a range of lithologies in the LaHood Formation (conglomerates to arkoses to siltstones) show that these units do not share a common provenance. The dominant provenance is Paleoarchean for the LaHood Formation and Paleoproterozoic for the Neihart Quartzite. Provenance is further constrained by the geochemistry and U-Pb ages of zircons from cobbles from the classic LaHood conglomerate in Jefferson Canyon (Tobacco Root Mountains), ages of Paleoproterozoic crystalline basement in the Beaverhead-Tendoy Mountains (1.8–2.45 Ga), and elemental and Sm-Nd isotopic data for select samples of both sedimentary rocks and crystalline basement within the basin. These data show a pronounced lack of detritus from abundant, proximal Neoarchean (2.7–2.9 Ga) and Paleoproterozoic (1.9–2.5 Ga) crystalline basement exposed in Laramide uplifts and the soles of Sevier-style thrust faults within and near the basin. Analyses of detrital mineral assemblages in the Lower Belt Supergroup units clearly indicate that the finer-grained portions of the LaHood Formation were not locally derived, based on abundant white mica in sections overlying tonalite-trondhjemite-granodiorite (TTG) basement and lack of amphibole in units overlying hornblende tonalites. Significant fractionation also exists between sand- and cobble-size components in conglomerate of the LaHood Formation in terms of elemental abundances, isotopic compositions, and the U-Pb ages of zircons. Stratigraphically, the differences in the ages of the youngest zircons in all LaHood Formation samples and the Neihart Quartzite (1.71 Ga, Neihart; 1.78 Ga, LaHood) do not refute any proposed stratigraphic correlations. Nonetheless, age spectra of detrital zircons from the Neihart Quartzite, all LaHood lithologies, and previously published data for the Newland Formation show distinctions of provenance and an apparent lack of interaction among the sediment-supply systems of these three formations. This contrast suggests that distinct, likely fault-bounded, sedimentologically restricted subbasins characterized the initial stages of development of the eastern Belt Basin along the Perry line (southeastern margin of the Helena embayment), in the manner of a modern, but partially submerged, Basin and Range topography. The time of development of this topography is not clear, but it may have been related to the collapse phase of the Great Falls orogeny at ca. 1.7 Ga for the Helena embayment. The primary, north-south–trending Belt Basin also developed subsequent to the Great Falls orogeny along the western paleomargin of the newly amalgamated Wyoming–Medicine Hat–Hearne craton.

The Pine Mountain window contains the southernmost Grenville basement massif to be found in the Appalachians. Granulite- and upper-amphibolite-facies granitic gneisses that form the basement complex are isotopically dated at 1.1–1.0 Ga. Locally, the gneisses contain rare mafic injections and supracrustal and plutonic xenoliths. The Pine Mountain Group cover sequence nonconformably overlies Grenville basement and is interpreted to correlate with Blue Ridge units as follows: Halawaka/Sparks Schist = Ocoee Supergroup (Late Proterozoic, rift), Hollis Quartzite = Chilhowee Group (Late Proterozoic-Cambrian, rift-to-drift), and Chewacla Marble = Shady Dolomite (Cambro-Ordovician, drift). Facies variations within the sedimentary cover units were cited as evidence for a southward decrease in the extent of the Ocoee rift basins, but new mapping documents the continuity of thick packages of Halawaka (i.e., Ocoee) rocks extending southward beneath the Gulf Coastal Plain. In contrast to upper amphibolite- and granulite-facies metamorphism of the basement during the Grenville event, cover rocks contain staurolite and staurolite-kyanite zone assemblages reflecting Paleozoic Appalachian metamorphism. Sensitive high-resolution ion microprobe (SHRIMP) and conventional single-grain U-Pb datings of detrital zircons from the basal Hollis Quartzite document a distinct population of clear, subrounded zircons of ca. 1.09 Ga, which were most likely derived from underlying Grenville-age gneiss. An older, white/gray population found in the lowermost Hollis is ca. 2.4–2.3 Ga, an age restricted to Gondwanan continents and very limited occurrences in northern Laurentia. Tectonic reconstructions of Unrug (1997) and others depict southeast Laurentia proximal to the Amazonia and Rio de la Plata cratons during the Neoproterozoic, offering the possibility that they may be the source for 2.4–2.3-Ga zircons in Hollis sediments. Alternatively, the AUSWUS (Australia/Western United States) reconstruction (Karlstrom et al., 2001) places east Antarctica and the Australian Gawler craton, both of which contain abundant 2.4 Ga granites, proximal to the southwestern United States during this time. Depending on the stream systems present during the Neoproterozoic, zircons from the Gawler may have been transported to the vicinity of the Pine Mountain window. In addition, three clear zircons yield ages of 1.4 Ga, and may have been derived from either the Laurentian Mid-continent granite-rhyolite province or the Rondonian Province of South America. A Chilhowee Group sandstone sample contains a similar mixture of Grenville and Mid-continent/Rondonian-age zircons, but none with ages of 2.4–2.3 Ga.