Michael Oates

Are we now living in the Anthropocene

The term Anthropocene, proposed and increasingly employed to denote the current interval of anthropogenic global environmental change, may be discussed on stratigraphic grounds. A case can be made for its consideration as a formal epoch in that, since the start of the Industrial Revolution, Earth has endured changes sufficient to leave a global stratigraphic signature distinct from that of the Holocene or of previous Pleistocene interglacial phases, encompassing novel biotic, sedimentary, and geochemical change. These changes, although likely only in their initial phases, are sufficiently distinct and robustly established for suggestions of a Holocene–Anthropocene boundary in the recent historical past to be geologically reasonable. The boundary may be defined either via Global Stratigraphic Section and Point (“golden spike”) locations or by adopting a numerical date. Formal adoption of this term in the near future will largely depend on its utility, particularly to earth scientists working on late Holocene successions. This datum, from the perspective of the far future, will most probably approximate a distinctive stratigraphic boundary.

Stratigraphy of the Anthropocene

The Anthropocene, an informal term used to signal the impact of collective human activity on biological, physical and chemical processes on the Earth system, is assessed using stratigraphic criteria. It is complex in time, space and process, and may be considered in terms of the scale, relative timing, duration and novelty of its various phenomena. The lithostratigraphic signal includes both direct components, such as urban constructions and man-made deposits, and indirect ones, such as sediment flux changes. Already widespread, these are producing a significant ‘event layer’, locally with considerable long-term preservation potential. Chemostratigraphic signals include new organic compounds, but are likely to be dominated by the effects of CO2 release, particularly via acidification in the marine realm, and man-made radionuclides. The sequence stratigraphic signal is negligible to date, but may become geologically significant over centennial/millennial time scales. The rapidly growing biostratigraphic signal includes geologically novel aspects (the scale of globally transferred species) and geologically will have permanent effects.

The lower Lias Group of the Hebrides Basin

Synopsis The lower and greater part of the Lias Group of the Hebrides Basin comprises shallow-marine to marginal-marine mudstones, siltstones, sandstones and limestones arranged in unambiguous transgressive-regressive facies cycles. In the northern region of the basin, the strata were deposited in a series of half graben bounded on their western margins by major eastward-dipping normal faults developed principally during the Triassic. Interpretation of the palaeogeographic setting of the lower Lias is complicated because exposure of the sequences occurs in the vicinity of transfer zones that have subsequently acted as foci for Palaeocene igneous activity. The principal exposures have been measured or remeasured in this study, and detailed sedimentary logs are presented that incorporate previously unpublished ammonite determinations. In the northern part of the Inner Hebrides Trough (Skye–Pabay–Raasay area), two formations are recognized: the Broadford Formation and the Pabay Shale Formation. The Broadford Formation, as used in this study, is equivalent only to the ‘Lower Broadford Beds’ of previous authors. The Pabay Shale incorporates the former ‘Upper Broadford Beds’ and ‘Pabba Shale’ of previous usage. Two new predominantly arenaceous members of the Pabay Shale Formation are formally defined for the northern area: the Hallaig Sandstone Member (Lower Sinemurian; Semicostatum–Turneri Zones) occurs within the lower Pabay Shale and has its type locality at Hallaig Waterfall on Raasay; the Suisnish Sandstone Member (Upper Sinemurian–Lower Pliensbachian; Raricostatum – Jamesoni zones) occupies the middle part of the Pabay Shale Formation and has its type locality at Rubha Suisnish and in the cliffs along the southern shore of Loch Eishort, on Skye. In the southern area (Mull–Morvern) use of the terms ‘Lower Broadford Beds’ or ‘Upper Broadford Beds’ is also inappropriate. In Ardnamurchan, and to a limited extent in more northerly exposures, the Broadford Formation interdigitates with its more southerly offshore equivalent, the Blue Lias Formation. The Pabay Shale of the Ardnamurchan–Morvern–Mull area is recognized down to the junction with the Blue Lias. A sandstone unit is well developed at about the Lower Sinemurian–Upper Sinemurian boundary: it is here named the Torosay Sandstone Member of the Pabay Shale Formation and is incompletely exposed at several localities on the southeastern coast of Mull. Although no age-diagnostic fossils have been found within the sandstone, it must lie within the upper Obtusum Zone to lower Raricostatum Zone interval. Overall, there is a general trend from more proximal environments of deposition in the north and east, towards more distal environments in the south and west. This regional pattern is modulated by the localized occurrence of sand-rich, very proximal successions in the vicinity of the supposed transfer zones, such as the area immediately SE of the northern termination of the Camasunary Fault. Records from the Upper Glen-1 borehole, in NW Skye, show that the expanded lower Lias sequence in the centre of the Sea of the Hebrides–Little Minch Basin can be subdivided in a similar manner to that exposed in the Inner Hebrides Trough; for example, it shows the development of more sandy strata at about the Lower Sinemurian to Upper Sinemurian boundary. Like the Ardnamurchan section, it also shows an intercalation of the Broadford Formation and Blue Lias. The upper half of the Pabay Shale appears to be truncated in the borehole: it is debatable whether this resulted from erosion prior to deposition of the Scalpa Sandstone, from sediment starvation or, alternatively, intersection of the borehole by a normal fault.

GSSPs, global stratigraphy and correlation

Abstract Procedures used to define an international chronostratigraphic stage boundary and to locate and ratify a Global Boundary Stratotype Section and Point (GSSP) are outlined. A majority of current GSSPs use biostratigraphic data as primary markers with no reference to any physico-chemical markers, despite the International Subcommission on Stratigraphic Classification (ISSC) suggestion that such markers should be included if possible. It is argued that such definitions will not produce the high-precision Phanerozoic time scale necessary to understand such phenomena as pre-Pleistocene ice ages and global climate change. It is strongly recommended that all GSSPs should have physico-chemical markers as an integral part of their guiding criteria, and where such markers cannot be found, the GSSP should be relocated. The methods and approach embodied in oceanic stratigraphy – coring, logging, analysing and archiving of drill sites by numerous experts using a wide range of methods – could usefully serve as a scientific model for the analysis and archiving of GSSPs, all of which are on the present-day continents. The incorporation of many more stratigraphic sections into GSSP studies, the application of physico-chemical methods, and the replacement of old U–Pb dates by newer CA-TIMS U–Pb dates, together with the use of constrained optimization (CONOP) programs that produce a calendar of events from many sections, should lead to much more precise timescales for pre-Cenozoic time than are currently available.

The lower Lias Group of the Hebrides Basin Published in Scottish Journal of Geology, Vol. 34(1), 1998, pp. 23–60

N. Morton writes: I congratulate Hesselbo et al. (1998) on an excellent paper on the lower Lias of the Hebrides which is a valuable contribution to the literature on the Jurassic of the area. I have some comments (1-6 below) which clarify or amplify some aspects of their paper, and I disagree with their proposed lithostratigraphical classification (see 7 below). Their usages of the terms Broadford Formation and Pabay Shale Formation have different meanings from the traditional usage of Broadford Beds Formation and Pabba Shale Formation, restricting one to only part of its original sense and extending the other. So, some care as to spelling and usage is required in reading the following discussion.