Shane Tyrrell

Drainage reorganization during breakup of Pangea revealed by in-situ Pb isotopic analysis of detrital K-feldspar

Pb isotopes in detrital K-feldspar grains provide a powerful provenance tracer for feldspathic sandstones. Common Pb isotopic compositions show broad (hundred-kilometer scale) regional variation, and this signature can survive weathering, transport, and diagenesis. The feldspar Pb signature can be measured rapidly using laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS), and careful targeting can avoid inclusions and altered regions within grains. Here, we combine a new Pb domain map for the circum–North Atlantic with detrital K-feldspar Pb isotopic data from Triassic and Jurassic sandstones from basins on the Irish Atlantic margin. The Pb isotopic compositions reveal otherwise cryptic feldspar populations that constrain the evolving drainage pattern. Triassic sandstones originated from distant Archean and Paleoproterozoic rocks, probably in Green-land, Labrador, and the Rockall Bank to the NW, implying long (>500 km) transport across a nascent rift system. Later, Jurassic sandstones had a composite Paleo- and Mesoproterozoic source in more proximal sources to the north (

Large-scale, linked drainage systems in the NW European Triassic: insights from the Pb isotopic composition of detrital K-feldspar

Pb isotopic data from K-feldspars in Middle Triassic (Anisian) sandstones in the Wessex Basin, onshore SW UK, and the East Irish Sea Basin, some 350 km to the north, show that the same grain populations are present. This indicates that the drainage system (the ‘Budleighensis’ river) feeding these basins originated from the same source/s, most probably the remnant Variscan uplands to the south. Fluvial and aeolian sandstones have the same provenance, suggesting that if water- and wind-driven sands were originally derived from different sources, this has been obscured through reworking prior to final deposition. Significant recycling of feldspar from arkosic sandstones in earlier sedimentary basins can be ruled out. The provenance data agree with previous depositional models, indicating transport distances in excess of 400 km, with a drainage pattern that linked separate basins. This supports the idea that the regional fluvial system was driven by topography and episodic flooding events of sufficient magnitude to overcome evaporation and infiltration over hundreds of kilometres. Importantly, this drainage system appears to have been isolated and independent from those operating contemporaneously to the NW of the Irish and Scottish massifs, where the remnant Variscan uplands apparently exerted no influence on drainage or sand supply.

First results from shallow stratigraphic boreholes on the eastern flank of the Rockall Basin, offshore western Ireland

The results of an integrated sedimentological and seismic stratigraphical analysis of three borehole sites on the eastern flank of the Rockall Basin, offshore western Ireland are reported. Two sites were drilled on the western slope of the Porcupine High, above the North and South Brona basins (boreholes 83/20-sb01, 83/24-sb01 and 83/24-sb02), and one on the northern flank of the Porcupine High (16/28-sb01), above the Macdara Basin. The cores establish that the half-graben basins marginal to the eastern Rockall Basin contain Jurassic deposits and that they were inverted sometime in the Late Jurassic or Early Cretaceous. An angular unconformity above the Brona basins is overlain by a condensed, tripartite Cretaceous succession (‘brownsand’, ‘greensand’, chalky micrite) that records stepwise deepening, with evidence for a Cenomanian-Turonian phase of normal faulting. Above the Macdara Basin, the unconformity is overlain by a basalt that was cored at the 16/28 site and is interpreted to represent a flow of Cretaceous age derived from the Drol Igneous Centre. At all three borehole sites, Cretaceous strata are onlapped (or downlapped) by Paleocene-Eocene strata that display evidence of a minor episode of fault reactivation above the Brona basins. Cored Eocene strata vary from clastic to carbonate-prone from north to south and smectitic clays are common at the 16/28 site. Post-Mid-Eocene westward tilting of the Rockall slope rotated the Eocene stratigraphy and the underlying Cretaceous deposits (including the lava flow in the 16/28 area) at least 3° down to the west. Slope development resulted in extensional sliding and the erosion of the C30 deep-water unconformity that is onlapped by Miocene slope deposits. C30 was cored in the 83/20 area where it cuts down into Cretaceous strata and is crusted with phosphates and the Cretaceous beneath Mn-impregnated.

Pb isotopic domains from the Indian Ocean sector of Antarctica: implications for past Antarctica–India connections

Abstract New feldspar lead isotope compositions of crystalline rocks from the Indian Ocean sector of East Antarctica, in conjunction with the review of data from elsewhere within the continent and from continents formerly adjacent within Gondwana, refine boundaries and evolutionary histories of terranes previously inferred from geological mapping and complementary isotope studies. Coastal Archaean Vestfold and Napier complexes have overlapping compositions and had Pb isotopes homogenized at 2.5 Ga sourced from or within already fractionated protoliths with high and variable U–Pb. Identical compositions from the Dharwar Craton of India support a correlation with these Antarctic terranes. The Proterozoic–Palaeozoic Rayner Complex and Prydz Belt yield more radiogenic compositions and are broadly similar and strongly suggest these units correlate with parts of the Eastern Ghats Belt of India. A strikingly different signature is evident from the inboard Ruker Complex, which yielded unradiogenic compositions. This complex is unlike any unit within India or Australia, suggesting that these rocks represent exposures of an Antarctic (Crohn) Craton. Compositions from the enigmatic Rauer Terrane are consistent with a shared early history with the Ruker Complex but with a different post-Archaean evolution. Supplementary material: Feldspar LA-ICP-MS Pb isotope data are available at www.geolsoc.org.uk/SUP18622

Sedimentology, sandstone provenance and palaeodrainage on the eastern Rockall Basin margin: evidence from the Pb isotopic composition of detrital K-feldspar

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.

Inferring sites of subglacial erosion using the Pb isotopic composition of ice-rafted feldspar: Examples from the Weddell Sea, Antarctica

The delivery of ice-rafted debris (IRD) from glaciated margins is a function of ice sheet dynamics. Shifts in supply and sourcing of IRD can therefore identify episodes of ice sheet instability; however, records can be difficult to correctly interpret because the subglacial geology of the catchment areas, which controls IRD composition, may be obscured. Importantly, variations can also result from shifts in erosion sites due to changes in the basal ice sheet conditions. This study evaluates where subglacial erosion has occurred in catchments that flow into the southern Weddell Sea, Antarctica, by determining the Pb isotopic compositions of individual ice-rafted feldspars from late Holocene marine sediments. Feldspar compositions match those of rock units inferred (through extrapolation of outcrop, magnetic, and gravity data) to compose areas where ice velocity, bed roughness, and shear stress are high. Significantly, signals from areas where ice velocities are high but bed roughness and shear stresses are low were not recorded, suggesting that there is reduced bedrock erosion in these regions. Major variations in IRD composition in the Weddell Sea can result from changing the loci of subglacial erosion, and do not necessarily correspond with major ice sheet instability.

Pb isotope compositions of detrital K‐feldspar grains in the upper‐middle Yangtze River system: Implications for sediment provenance and drainage evolution

The upper-middle Yangtze River drains the Qiangtang Block, the Songpan-Ganzi, the Yangtze Craton, and the Qinling-Dabie orogenic belt. These tectonic units have been shown to have heterogeneous Pb isotopic compositions, which allow this isotope system to be used as a sediment provenance tool. In this study we have employed laser ablation multiple collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) to measure Pb isotope compositions of sand-sized K-feldspar grains from the upper-middle Yangtze River. Data are presented from four major tributaries: Yalongjiang, Minjiang, Jialingjiang, and Hanjiang, as well as from the main Yangtze River near Yichang. A portion of K-feldspar grains in the Yalongjiang shows an ultraradiogenic character (206Pb/204Pb > 20), which is unique in the upper-middle Yangtze. Moreover, these ultraradiogenic grains were transported as far as Yichang, just downstream from the Three Gorges, suggesting that the Pb-in-K-feldspar method could be applied to the sediments within the Jianghan Basin to date the formation of the Three Gorges. Pb isotopic data from Yichang indicate that erosion in the Longmen Shan and neighboring regions is more important than the Jinshajiang in supplying sediment. The grains in Hanoi Basin have little overlap with the Songpan-Ganzi, but show a good match with the Yangtze Craton in its range of lower 206Pb/204Pb ratios. These observations support the idea that the “Middle Yangtze” used to be a tributary of the paleo-Red River and that there has been no drainage linking the Songpan-Ganzi and the Red River since the Eocene.

Provenance of detrital K-feldspar in Jianghan Basin sheds new light on the Pliocene–Pleistocene evolution of the Yangtze River

The Yangtze River is the largest fluvial system draining the Tibetan Plateau, yet its time of formation—pre-Miocene versus Pleistocene—has been debated for more than a century, with a particular focus on the incision of the Three Gorges. In this study, we used laser-ablation−multiple collector−inductively coupled plasma−mass spectrometry (LA-MC-ICP-MS) to measure Pb isotope compositions of detrital K-feldspars from Pliocene−Pleistocene sediments in the Jianghan Basin, located just downstream of the Three Gorges. Our new Pb results indicate that feldspars from the Songpan-Ganzi terrane were already being delivered to the Jianghan Basin by ca. 3.4 Ma. Therefore, we suggest that the Three Gorges was incised prior to the late Pliocene. Moreover, the Pb isotopic data also record the first delivery of detritus from the Hanjiang River, one of the Yangtze’s largest tributaries in the middle reaches, into the Jianghan Basin at ca. 1.8 Ma. Most K-feldspar grains from the mid-late Pleistocene samples have similar Pb isotopic compositions to those from the major tributaries in the upper reaches, i.e., Minjiang and Jialingjiang Rivers, indicating that the eastern Tibetan Plateau, as the major sediment supplier to the Jianghan Basin and mid-lower Yangtze River, has played a key role in the sediment source-to-sink transport of this river. This study sheds light on the factors that control the supply of detrital K-feldspar in large-scale drainage systems.

Sedimentary provenance constraints on drainage evolution models for SE Tibet: Evidence from detrital K-feldspar

The unusual drainage pattern of the Jinsha River (upper Yangtze) is closely linked to the uplift of the Tibetan Plateau, but when this pattern became established is controversial. In this study we compare the Pb isotopic composition of detrital K-feldspars from the modern Jinsha River with those from the Plio-Pleistocene Xigeda Formation, a sequence of fluvial and lacustrine deposits widely distributed along the major rivers draining the Tibetan Plateau, to investigate their relationship and to constrain the drainage evolution of the region. Our results demonstrate that the Jinsha River had achieved its current sediment-transport pattern by the late Pliocene, indicating a drainage character similar to that operating at present. Our findings not only suggest an upper age limit for formation of the Jinsha River but also shed new light on the evolution of the whole Yangtze system. K-feldspar grains from the Jinsha River have less radiogenic Pb than those in the Red, Mekong, and Salween Rivers, providing a key test as to whether the Jinsha River was previously connected to the paleo-Red River. Comparison with published Pb isotopic data shows that these characteristically unradiogenic feldspars are absent in the onshore Cenozoic archive from the Hanoi Basin, suggesting that there has been no connection between the lower Jinsha River and the Red River since the Eocene.

Sand supply to the Lake Albert Basin (Uganda) during the Miocene‐Pliocene: A multiproxy provenance approach

A multi-proxy provenance approach (heavy mineral analysis, U-Pb zircon geochronology and Pb isotopic analysis of K-feldspar) has constrained sediment supply within the Upper Nile drainage system in the Miocene - Pliocene. Provenance data from sandstones were obtained from three exploration wells, two situated on the north-eastern margin and one on the eastern flank of the Lake Albert Basin, NW Uganda. Data suggest that high- to low-grade metamorphic rocks and granitoids have variably supplied the heavy mineral assemblages around the Lake Albert Basin during the Miocene-Pliocene, with contributions from the isotopically-heterogeneous Archean Cratons (including the local Ugandan Craton, Tanzanian and Congo Cratons) and the Pan-African rocks (the Mozambique Belt) with possible contributions from the Neoproterozoic and Paleoproterozoic rocks. These data also highlight clear differences between supply to the eastern basin margin, compared with the northeast, which is reconcilable with current models for Miocene-Pliocene drainage in the region. Supply to northeastern Lake Albert during the Miocene-Pliocene appears to have been through a proto-Albert Nile (draining from NE to SW) and from a proto-Victoria Nile or similarly oriented palaeo-river systems draining from the east. In contrast, the eastern flanks of the basin were likely supplied via the palaeo-Nkusi river, tapping local hinterland sources and more distal basement to the far-east (Mozambique Belt). This study highlights the importance of utilizing a multi-proxy approach in provenance analysis as no one signal is capable of distinguishing the different source lands and constraining the evolving drainage patterns.