ABSTRACT In 1935, Krynine postulated that first-cycle arkose in the humid tropical setting of southern Mexico can be rapidly eroded with minimal chemical weathering and redeposited as second-cycle arkose. Modern quantitative data confirm this hypothesis and highlight exceptions where first-cycle arkosic sediments have been diagenetically altered by intense weathering to yield second-cycle quartz arenites. In this study, extensive sampling of upland source rocks and their derived sediments provided a robust data set with which to quantitatively evaluate the composition and provenance of Holocene sediments. Three upland source terrains were identified: Paleozoic crystalline basement of the Chiapas Massif; Mesozoic to Cenozoic siliciclastic and carbonate rocks of the Chiapas fold belt; and Cenozoic sedimentary rocks in the foothills of the fold belt. Holocene sediments from these source terrains are grouped into seven facies (A–G) based on their provenance and geographic location. Facies A consists of feldspathic sediments from the Mezcalapa-Grijalva River that are sourced from the Chiapas Massif. Facies B consists of lithic-rich sediments from the same area that are derived from the Chiapas fold belt. Facies A and B consist predominantly of first-cycle sand capable of yielding arkosic deposits. Facies C represents a mixture of Facies A and B sands deposited along the course of the Mezcalapa-Grijalva River. Facies D (from Rio Sierra) and Facies E (from Rio Pedregal) represent second-cycle feldspathic sands of the coastal-plain delta and were derived from Cenozoic sedimentary rocks of the foothills. Mild chemical weathering due to rapid mechanical erosion enabled the creation of these arkosic deposits. They are less feldspathic than their parents and have limited occurrence due to mixing with less feldspathic first-cycle sands downstream from their sources. Facies F (from Rio Zanapa) and Facies G (from Lagunas Rosario and Enmedio) represent second-cycle quartzose sands of the low-lying savanna that were also derived from Cenozoic sedimentary rocks in the foothills of the fold belt. Intense, long-term (>10,000 yr) chemical weathering of these sands has precluded the formation of arkoses, instead yielding quartz arenites. They are more weathered than the delta sands (Facies D, E) with a greater loss of feldspar and carbonate detritus. They are enriched in silica and depleted in alumina, CaO, Na 2 O, and K 2 O relative to Facies A arkoses due to loss of feldspars and mafic minerals. Second-cycle sediments eroded from Tertiary sedimentary rocks in the foothills (Facies D–G) contain detrital serpentine and chromite with high abundances of Cr and Ni, suggesting an ultramafic component in their provenance. Cr and Ni are effective tracers for second-cycle components in sands of mixed provenance.

Abstract A number of rock samples were collected from two dredge positions on the Lomonosov Ridge at water depths of 2–3.5 km. The dredge samples are dominated by sediments deformed and metamorphosed under greenschist-facies conditions 470 myr ago according to 40 Ar/ 39 Ar dating of metamorphic muscovite. This shows that the Lomonosov Ridge was involved in a major Mid-Ordovician orogenic event that correlates with early arc–terrane accretion observed in northern Ellesmere Island, Svalbard, and other parts of the Caledonian belt. Detrital zircon age spectra of these metasediments span the Mesoproterozoic–Palaeoproterozoic with a main peak at around 1.6 Ga, and a pattern similar to that known from Caledonian metasedimentary rocks in East Greenland and northern Norway, as well as from Cambrian sediments in Estonia and Palaeozoic sediments on Novaya Zemlya. A second population of dredge samples comprises undeformed, non-metamorphic sandstones and siltstones. Detrital zircons in these sediments span the Palaeoproterozoic with a few Archaean zircons. Both rock types are covered by an up to 8 Ma ferromanganese crust and are evaluated to represent outcrop, and apatite fission-track data from three of the rock samples indicate that exposure at the seabed corresponds to a regional event of uplift and erosion that affected the Arctic in the Late Miocene. The data from the Lomonosov Ridge suggest that the 470 Ma orogenic event extended from Scotland and northern Scandinavia into the Arctic, including Svalbard, the Pearya Terrane and the Chukchi Borderlands. Supplementary material: Detrital zircon age data and details of the thermal history constraints are available at: https://doi.org/10.6084/m9.figshare.c.3852151

The Arizaro Basin in northwestern Argentina sits today in the western Puna Plateau at elevations of 3800–4200 m along the eastern flank of the Miocene to modern magmatic arc. The basin is roughly circular in plan view and ~100 km in diameter, and it was filled during Miocene time (ca. 21–9 Ma) by >3.5 km of eolian, alluvial, fluvial, and lacustrine sediment in addition to ash-fall tuffs from the Andean magmatic arc. The basin fill was subsequently shortened in its central part, and it has been uplifted and topographically inverted. The Arizaro Basin is not obviously related to known faults, nor does it exhibit a peripheral belt of coarse-grained sedimentary rocks derived from flanking topographically higher regions. Sandstone modal framework compositions are arkosic, but not as rich in volcanic lithic fragments as typical intra-arc basins. Detrital zircon U-Pb age spectra implicate source terranes in locally exposed Ordovician granitoid rocks, more distal Upper Paleozoic–Mesozoic arc terranes in western Argentina and possibly northern Chile, and the local Miocene magmatic arc. Depositional-age zircons are present in most of the sandstones analyzed for detrital zircon U-Pb geochronology, and zircon U-Pb ages from volcanic tuff layers provide independent chronological control. The tectonic component of subsidence initiated at low rates, accelerated to ~0.6 mm/yr during the medial stage of basin development, and tapered off to zero as the basin began to shorten internally and experience topographic inversion after ca. 10 Ma. Together, the data presented here suggest that the Arizaro Basin could have developed in response to the formation and gravitational foundering of a dense Rayleigh-Taylor–type instability in the lower crust and/or mantle lithosphere. Insofar as hinterland basins of uncertain tectonic affinity are widespread in the high central Andes, the model developed here may be relevant for other regions of enigmatic subsidence and sediment accumulation in the Andes and other cordilleran hinterland settings.

The King Ridge Road mélange is a unit of the Franciscan Complex, cropping out in an area of at least 50 km 2 around the town of Cazadero, coastal California. This unit is an olistostrome with a massive, unfoliated sandstone matrix, containing >232 large meta-igneous and chert blocks of greatly varying size, lithology, and metamorphic history within the study area. This sandstone matrix is litharenite or arkosic arenite and exhibits prograde prehnite-pumpellyite facies and retrograde zeolite facies metamorphism. It is devoid of megascopic textures except for rare simple bedding. No fossils have been found, and no Bouma units or other graded beds are present. Detrital zircon geochronology has established the maximum age of deposition of the sandstone matrix at 83 Ma, whereas apatite fission-track data indicate cooling of the olistostrome below 100 °C at ca. 35–38 Ma. The 232 exotic blocks sampled in the study area are dominantly low- to medium-grade greenstones and cherts, together with fewer high-grade blocks partly composed of blue amphibole and/or omphacitic pyroxene, and some amphibolites. Thus, many of the blocks have higher grade metamorphic assemblages than the matrix. All block types are well mixed together, so none greatly predominate anywhere. Blocks of oceanic-island-arc plutonic rocks, including granitoids and recemented breccias, are particularly distinctive for this mélange. One granitoid block has a zircon U-Pb age of 165 ± 1 Ma. The massive sandy matrix of the olistostrome formed by accumulation of hyperconcentrated sedimentary density flows (grain flows) sourced primarily from the Klamath-Sierra continental magmatic arc. Many of the blocks record a pre-mélange history of metamorphism and exhumation, followed by partial subduction and reburial with the matrix after 83 Ma. Cooling below 100 °C took place at 35–38 Ma, probably associated with partial exhumation of the unit, with subsequent removal of ~10 km of cover.

Abstract The Rockall Basin, west of Ireland, is a frontier area for hydrocarbon exploration, but currently the age and location of sand fairways through the basin are poorly known. A recently developed provenance approach based on in-situ Pb isotopic analysis of single K-feldspar grains by laser ablation multi-collector inductively coupled mass spectrometry (LA-MC-ICPMS) offers advantages over other provenance techniques, particularly when applied to regional palaeodrainage issues. K-feldspar is a relatively common, usually first-cycle framework mineral in sandstones and its origin is typically linked to that of the quartz grains in arkosic and sub-arkosic rocks. Consequently, in contrast to other techniques, the Pb-in-K-feldspar tool characterizes a significant proportion of the framework grains. New Pb isotopic data from K-feldspars in putative Permo-Triassic and Middle Jurassic sandstones in Well 12/2-1z (the Dooish gas condensate discovery) on the eastern margin of the Irish Rockall Basin are reported. These data suggest that three isotopically distinct basement sources supplied the bulk of the K-feldspar in the reservoir sandstones and that the relative contribution of these sources varied through time. Archaean and early Proterozoic rocks (including elements of the Lewisian Complex and its offshore equivalents), to the immediate east, NE and north of the eastern Rockall Margin, are the likely sources. More distal sourcelands to the NW cannot be ruled out but there was no significant input from southern sources, such as the Irish Massif. These data, together with previously published regional Pb isotopic data, highlight the important role played by old, near and far-field Archaean–Proterozoic basement highs in contributing sediment to NE Atlantic margin basins. The Irish Massif appears to have acted as a significant, but inert, drainage divide from the Permo-Triassic to the Late Jurassic and hence younger, Avalonian and Variscan sand sources appear to have been less important on the Irish Atlantic Margin.

Abstract Eocene nonmarine sedimentary rocks that occur in northwest and central Washington as a widespread series of outcrops are evidence of a meandering river system that existed prior to the mid-Tertiary uplift of the North Cascade Range. Arkosic strata appear to have initially been deposited in a basin that was later divided by strike-slip faulting, producing outcrops of the Swauk Formation on the eastern flank of the North Cascades, and the Chuckanut Formation to the west. Plant fossils are abundant in both formations, but the Swauk paleoflora has received little study. The Chuckanut Formation paleoflora records a marked shift in the region’s paleoclimate. The Late Paleocene to Middle Eocene Bellingham Bay and Slide Stratigraphic Members, which comprise the lower 6000 m of the formation, contain diverse assemblages of subtropical plant fossils. In contrast, the overlying 3000-m-thick Padden Member contains taxa indicative of a warm temperate paleoclimate. An unconformity may separate the Padden Member from older Chuckanut strata, and the age of the Padden Member has not been determined. The climate shift may have been a Late Eocene fluctuation, but the possibility that the floral changes represent the transitional Eocene-Oligocene cooling event cannot be discounted. Animal fossils from the Chuckanut Formation include aquatic mollusks and a soft-shelled turtle, and track impressions from a variety of birds and mammals.