The debate concerning the origin of the Whin Sill of northeast England during the early and mid-19th century

The term sill nowadays employed for a broadly concordant igneous intrusion is widely believed to have been derived from the local term for persistent hard beds in the Carboniferous sequence of Northeast England, in particular the Whin Sill. Despite the intrusive origin of the Whin having been demonstrated in Teesdale by Sedgwick in 1827, for much of the 19th Century the alternative extrusive hypothesis, of which Phillips (1836) was the principal proponent, was widely favoured. There were three principal reasons why the intrusive origin was not more widely held, unquestioning acceptance of the erroneous belief of local miners that the Whin Sill was always at the same stratigraphical horizon, a perception that the Teesdale outcrops were not necessarily typical of the rest of the region and a reluctance to accept that the intrusion of such large volumes of magma over such a great area was physically possible. In the 1870s, first the work of Tate and then the detailed six-inch to one-mile mapping of the Geological Survey finally dispelled any notion that the sill was at a consistent stratigraphical level. Curiously, though the correct determination of the intrusive origin of the Whin Sill was one of Sedgwick's earliest and greatest achievements in Northern England, it was not deemed of sufficient importance to merit mention by his biographers.

Impact of igneous intrusion and associated ground deformation on the stratigraphic record

The geomorphology and sediment systems of volcanic areas can be influenced by uplift (forced folding) related to subsurface migration and accumulation of magma. Seismic geomorphological analysis presents a unique tool to study how surface morphology and subsurface magma dynamics relate, given seismic reflection data can image buried landscapes and underlying intrusions in 3D at resolutions of only a few metres-to-decametres. However, differential compaction of the sedimentary sequence above incompressible igneous intrusions during burial modifies palaeosurface morphology. Here we use 3D seismic reflection data from offshore NW Australia to explore how the stratigraphic record of igneous intrusion and associated ground deformation can be unravelled. We focus on a forced fold that formed in the Early Cretaceous to accommodate intrusion of magma, but which was later amplified by burial-related differential compaction of the host sedimentary sequence. We show how: (1) marine channels and clinoforms may be deflected by syn-depositional intrusion-induced forced folds; and (2) differential compaction can locally change clinoform depth post-deposition, potentially leading to erroneous interpretation of shoreline trajectories. Our results demonstrate seismic geomorphological analysis can help us better understand how magma emplacement translates into ground deformation, and how this shapes the landform of volcanic regions.

Effects of Igneous Intrusion on the Organic Content of Irati Formation, Paraná Basin, in Sapopema (PR)

The thermal effects of an igneous intrusion on organic-rich sedimentary rocks can be considering an important source of maturation of organic matter. The Permian Irati Formation of Paraná Basin (Brazil) is a carbonatic and organic-rich shale sequence intruded by Jurassic-Cretaceous basic rocks. This study reports possible effects of igneous intrusion on the organic matter content of Irati Formation, in Sapopema region (Paraná State). Total organic carbon (TOC), total sulfur (S) and insoluble residue (IR) data were combined with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The relatively low TOC values recorded in well where Irati Formation is in contact with 60 m of diabase sill (SP-58-PR) are residuals, associated with the depletion of organic carbon, caused by the thermal effect from the overlying intrusive rock. It was responsible to promote the cracking of the organic matter and reduced those values in relation to the original ones, observed in SP-32-PR (without thermal influence). When comparing the TOC peaks of the Assisting Member in both wells, it was observed that there was a decrease between 80.7 and 84% in the SP-58-PR. SEM images reveal that organic matter in Taquaral Member is sub-rounded and regular shape, while the organic matter in Assistência Member presents a characteristic pattern of thermally evolved organic matter.

Transformation of minerals at the boundary of magma-coal contact zone: case study from Wolonghu Coal Mine, Huaibei Coalfield, China

Mesozoic and Cenozoic magma activity in the Wolong Lake mining area of Huaibei is frequent, and the degree of magma intrusion into coal seams remarkable. On the one hand, magma intrusion affects the utilization of coal resources; on the other hand, the macro and trace elements in coal are redistributed to form new mineral types. This study uses the Wolong Lake magma intrusion coal seam as a research object. The mineral paragenesis for igneous rock, coke, and thermally-altered coal in an igneous intrusion zone is studied using SEM, XRD, and Raman spectroscopy. During igneous intrusion, the temperature and pressure of igneous rock metamorphose ambient low-rank coal to high-rank coal and coke. The response mechanism of minerals and trace elements to magmatic intrusion is discussed. The results are: ① SEM analysis shows that ankerite and pyrite are formed from magma intrusion. Both minerals are strongly developed in the magma-coal contact zone, and less well developed in thermally-altered coal. ② XRD analysis shows that igneous intrusion strongly influences the types and content of minerals in coke and thermally-altered coal. In addition to the increase amounts of ankerite and pyrite, chlorite, serpentine, and muscovite, and other secondary minerals, are generated following igneous intrusion. ③ Raman analysis suggests that thermally-altered coal possesses the characteristics of both pyrite and coke. Coke from the magma-coal boundary zone possesses the typical characteristics of pyrite. Igneous rock contains a mineral similar to pyrite, confirmed by both having similar Raman peaks. The scattering intensity of Ag indicates that the formation pressure of pyrite increases from thermally-altered coal via the boundary between the coke zone and the igneous rock.