In late Mesozoic time, the southern Cordilleran foreland basin was bounded on the west by the Sevier thrust belt and on the south by the Mogollon highlands. Paleocurrent indicators in fluvial and fluviodeltaic strata imply sediment delivery into the basin from both tectonic features. Ages of detrital zircons in sandstones of the basin provide insights into the nature of the sediment sources. Upper Jurassic and Lower Cretaceous fluvial strata were deposited as sediment blankets across the width of the basin but Upper Cretaceous marginal-marine facies were restricted to the basin margin, with marine facies in the basin interior. Most Upper Jurassic and Lower Cretaceous fluvial sandstones contain heterogeneous age populations of Precambrian and Paleozoic detrital zircons largely recycled from Jurassic eolianites uplifted within the Sevier thrust belt or antecedent highlands, and exposed as sedimentary cover over the Mogollon highlands, with only minor contributions of Mesozoic zircon grains from the Cordilleran magmatic arc along the continental margin. Sources in Yavapai-Mazatzal Proterozoic basement intruded by anorogenic Mesoproterozoic plutons along the Mogollon highlands were significant for the Westwater Canyon Member of the Upper Jurassic Morrison Formation and for early Upper Cretaceous (Turonian) fluviodeltaic depositional systems, in which arc-derived Cordilleran zircon grains are more abundant than in older and younger units composed dominantly of recycled detritus. Detrital zircons confirm that the Salt Wash and Westwater Canyon Members of the Morrison Formation formed separate foreland megafans of different provenance. Late Upper Cretaceous (Campanian) fluvial sandstones include units containing mostly recycled sand lacking arc-derived grains in the Sevier foredeep adjacent to the Sevier thrust front, and units derived from both Yavapai-Mazatzal basement and the Cordilleran arc farther east, with some mingling of sand from both sources at selected horizons within the Sevier foredeep. Evidence for longitudinal as well as transverse delivery of sediment to the foreland basin shows that paleogeographic and isostatic analyses of thrust-belt erosion, sediment loads, and basin subsidence in foreland systems need to allow for derivation of foreland sediment in significant volumes from sources lying outside adjacent thrust belts.

The concept of a critical moment in a petroleum system (the time of highest probability of entrapment and preservation of oil and gas) has underlain petroleum exploration for over 25 years. However, one area where understanding the critical moment is challenging is the Faroe-Shetland Basin (FSB; offshore UK). Isotopic dating of oils suggests that petroleum generation began between ca. 68 and 90 Ma; however, most basin models invoke an earlier generation beginning in the mid-Cretaceous at ca. 100 Ma, predating deposition of Paleocene and Eocene reservoirs. This time discrepancy has previously been explained by remigration from intermediary accumulations ("motel" hypothesis) and/or overpressure retardation of kerogen maturation. The FSB is characterized by a thick Cretaceous stratigraphic package (up to 5 km) that includes a large net thickness (up to 2 km) of Paleogene igneous material. In our model, separating sedimentary and igneous material and adding the igneous material at the correct time between ca. 58 and 55 Ma shallows the modeled burial depth of the Upper Jurassic source rocks during the Cretaceous sufficiently to delay maturation by 17 m.y. in comparison to results of previous studies. Additionally, previous studies have invoked crustal radiogenic heat production (RHP) based on the Phanerozoic crust averaging ∼2.8 μW/m 3 in the North Sea (300 km to the east). However, the FSB basement is composed of significantly older, colder Neoarchean orthogneisses (ca. 2.7-2.9 Ga), reducing RHP by up to 50% to ∼1.6 μW/m 3 (σ = 0.74). For the first time, our model unifies geological, geochronological, and geochemical observations, delaying the onset of petroleum expulsion by up to 40 m.y. in comparison to previous models.

This study presents the results of a joint Chemostrat – APT study that aimed to produce a suite of Radioactive Heat Production (RHP) data for basement rocks in the Faroe Shetland Basin to enable more accurate basin modelling to be undertaken. To enable regional studies to be undertaken, the basement has been split into four zones based on similarities. Zone A is formed of high grade metamorphic basement from the Rockall trough (quads 154 & 164) southwest of the “Laxfordian front” postulated by Holdsworth et al., (2019). Zone B comprises granodioritic, tonalitic and dioritic Neoarchean aged (2700-2830 Ma) high grade metamorphic basement from the southwest of the Rona Ridge and Basin (wells 202/08-1, 204/15-2, 205/161, 205/21-1A, 206/7a-2, 206/08-2, 206/09-2 and 206/12-1) and northeast of the Laxfordian front. Zone C contains Neoarchean aged high grade metamorphic basement of a predominantly granitic and quartz rich granitoid composition from the northeast of the Rona Ridge (wells 207/01-3, 207/02-1, 208/23-1 and 208/26-1). Zone D differs from the rest of the material in this study in that it is Caledonian (∼460 Ma) granitic plutonic basement from Quads 209 (Ereland volcanic centre). Radioactive heat production values were derived from Potassium, Thorium and Uranium data produced from the analysis of eighty-four basement samples by ICP-OES and ICP-MS analysis. Each mapped basement zone was then assigned a mean radioactive heat production value for use in future basin modelling studies; Zone A = 0.21 µWm 3 , Zone B, 0.64 µWm 3 , zone C = 0.88 µWm 3 and zone D = 2.1 µWm 3 . Thematic collection: This article is part of the UKCS Atlantic Margin collection available at: https://www.lyellcollection.org/topic/collections/new-learning-from-exploration-and-development-in-the-ukcs-atlantic-margin Supplementary material: https://doi.org/10.6084/m9.figshare.c.6771540

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