The Great Unconformity is a widely distributed surface separating Precambrian rocks from overlying Phanerozoic sedimentary sequences. The causes and implications of this feature, and whether it represents a singular global event, are much debated. Here, we present new apatite (U‐Th)/He (AHe) thermochronologic data from the central Canadian Shield that constrain when the Precambrian basement last cooled to near‐surface temperatures, likely via exhumation, before deposition of overlying early Paleozoic sedimentary sequences that mark the Great Unconformity. AHe data from 11 samples (
Diachronous development of Great Unconformities before Neoproterozoic Snowball Earth
The Great Unconformity marks a major gap in the continental geological record, separating Precambrian basement from Phanerozoic sedimentary rocks. However, the timing, magnitude, spatial heterogeneity, and causes of the erosional event(s) and/or depositional hiatus that lead to its development are unknown. We present field relationships from the 1.07-Ga Pikes Peak batholith in Colorado that constrain the position of Cryogenian and Cambrian paleosurfaces below the Great Unconformity. Tavakaiv sandstone injectites with an age of ≥676 ± 26 Ma cut Pikes Peak granite. Injection of quartzose sediment in bulbous bodies indicates near-surface conditions during emplacement. Fractured, weathered wall rock around Tavakaiv bodies and intensely altered basement fragments within unweathered injectites imply still earlier regolith development. These observations provide evidence that the granite was exhumed and resided at the surface prior to sand injection, likely before the 717-Ma Sturtian glaciation for the climate appropriate for regolith formation over an extensive region of the paleolandscape. The 510-Ma Sawatch sandstone directly overlies Tavakaiv-injected Pikes granite and drapes over core stones in Pikes regolith, consistent with limited erosion between 717 and 510 Ma. Zircon (U-Th)/He dates for basement below the Great Unconformity are 975 to 46 Ma and are consistent with exhumation by 717 Ma. Our results provide evidence that most erosion below the Great Unconformity in Colorado occurred before the first Neoproterozoic Snowball Earth and therefore cannot be a product of glacial erosion. We propose that multiple Great Unconformities developed diachronously and represent regional tectonic features rather than a synchronous global phenomenon.
more » « less- NSF-PAR ID:
- 10146740
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 117
- Issue:
- 19
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- p. 10172-10180
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract n = 57) across a broad region define a similar date‐eU pattern, implying a common thermal history. Higher eU (>25 ppm) apatite form distinct flat profiles of reproducible dates at ∼510 ± 49 Ma (mean and 1σ standard deviation), while lower eU (<25 ppm) apatite define a positive date‐eU trend with younger dates. The data patterns, geologic context, and thermal history modeling point toward >3 km of erosion across the entire ∼450,000 km2study area between 650 and 440 Ma, followed by modest reheating during later burial. Plume activity associated with intracratonic basin formation or continental rifting/breakup may have caused this erosion event. The post‐650 Ma timing of the last major sub‐Great Unconformity exhumation phase in this region implies a late Great Unconformity that is younger than inferred elsewhere in North America. This suggests that this feature is likely the result of multiple temporally distinct erosion events with differing footprints and mechanisms. -
The origin of the phenomenon known as the Great Unconformity has been a fundamental yet unresolved problem in the geosciences for over a century. Recent hypotheses advocate either global continental exhumation averaging 3 to 5 km during Cryogenian (717 to 635 Ma) snowball Earth glaciations or, alternatively, diachronous episodic exhumation throughout the Neoproterozoic (1,000 to 540 Ma) due to plate tectonic reorganization from supercontinent assembly and breakup. To test these hypotheses, the temporal patterns of Neoproterozoic thermal histories were evaluated for four North American locations using previously published medium- to low-temperature thermochronology and geologic information. We present inverse time–temperature simulations within a Bayesian modeling framework that record a consistent signal of relatively rapid, high-magnitude cooling of ∼120 to 200 ° C interpreted as erosional exhumation of upper crustal basement during the Cryogenian. These models imply widespread, synchronous cooling consistent with at least ∼3 to 5 km of unroofing during snowball Earth glaciations, but also demonstrate that plate tectonic drivers, with the potential to cause both exhumation and burial, may have significantly influenced the thermal history in regions that were undergoing deformation concomitant with glaciation. In the cratonic interior, however, glaciation remains the only plausible mechanism that satisfies the required timing, magnitude, and broad spatial pattern of continental erosion revealed by our thermochronological inversions. To obtain a full picture of the extent and synchroneity of such erosional exhumation, studies on stable cratonic crust below the Great Unconformity must be repeated on all continents.more » « less
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Abstract The Great Unconformity of the Rocky Mountain region (western North America), where Precambrian crystalline basement is nonconformably overlain by Phanerozoic strata, represents the removal of as much as 1.5 b.y. of rock record during 10-km-scale basement exhumation. We evaluate the timing of exhumation of basement rocks at five locations by combining geologic data with multiple thermochronometers. 40Ar/39Ar K-feldspar multi-diffusion domain (MDD) modeling indicates regional multi-stage basement cooling from 275 to 150 °C occurred at 1250–1100 Ma and/or 1000–700 Ma. Zircon (U-Th)/He (ZHe) dates from the Rocky Mountains range from 20 to 864 Ma, and independent forward modeling of ZHe data is also most consistent with multi-stage cooling. ZHe inverse models at five locations, combined with K-feldspar MDD and sample-specific geochronologic and/or thermochronologic constraints, document multiple pulses of basement cooling from 250 °C to surface temperatures with a major regional basement exhumation event 1300–900 Ma, limited cooling in some samples during the 770–570 Ma breakup of Rodinia and/or the 717–635 Ma snowball Earth, and ca. 300 Ma Ancestral Rocky Mountains cooling. These data argue for a tectonic control on basement exhumation leading up to formation of the Precambrian-Cambrian Great Unconformity and document the formation of composite erosional surfaces developed by faulting and differential uplift.more » « less
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Abstract Our study used zircon (U-Th)/He (ZHe) thermochronology to resolve cooling events of Precambrian basement below the Great Unconformity surface in the eastern Grand Canyon, United States. We combined new ZHe data with previous thermochronometric results to model the <250 °C thermal history of Precambrian basement over the past >1 Ga. Inverse models of ZHe date-effective uranium (eU) concentration, a relative measure of radiation damage that influences closure temperature, utilize He diffusion and damage annealing and suggest that the main phase of Precambrian cooling to <200 °C was between 1300 and 1250 Ma. This result agrees with mica and potassium feldspar 40Ar/39Ar thermochronology showing rapid post–1400 Ma cooling, and both are consistent with the 1255 Ma depositional age for the Unkar Group. At the young end of the timescale, our data and models are also highly sensitive to late-stage reheating due to burial beneath ∼3–4 km of Phanerozoic strata prior to ca. 60 Ma; models that best match observed date-eU trends show maximum temperatures of 140–160 °C, in agreement with apatite (U-Th)/He and fission-track data. Inverse models also support multi-stage Cenozoic cooling, with post–20 Ma cooling from ∼80 to 20 °C reflecting partial carving of the eastern Grand Canyon, and late rapid cooling indicated by 3–7 Ma ZHe dates over a wide range of high eU. Our ZHe data resolve major basement exhumation below the Great Unconformity during the Mesoproterozoic (1300–1250 Ma), and “young” (20–0 Ma) carving of Grand Canyon, but show little sensitivity to Neoproterozoic and Cambrian basement unroofing components of the composite Great Unconformity.more » « less
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