Christopher W. Thomas

A composite C-isotope profile for the Neoproterozoic Dalradian Supergroup of Scotland and Ireland

Abstract: The Neoproterozoic Dalradian Supergroup is a dominantly siliciclastic metasedimentary succession in the Caledonian orogenic belt of Scotland and Ireland. Despite polyphase deformation and greenschist- to upper amphibolite-facies metamorphism, carbonate units distributed throughout the Dalradian record marked δ13Ccarbonate excursions that can be linked to those associated with key environmental events of Neoproterozoic time. These include: (1) tentative correlation of the Ballachulish Limestone with the c. 800 Ma Bitter Springs anomaly; (2) the presence of the pre-Marinoan Trezona anomaly and 635 Ma Marinoan-equivalent cap carbonate sequence in rocks of the middle Easdale Subgroup; (3) the terminal Proterozoic (c. 600–551 Ma) Wonoka–Shuram anomaly in the Girlsta Limestone on Shetland. These linkages strengthen previously inferred correlations of the Stralinchy–Reelan formations and the Inishowen–Loch na Cille–MacDuff ice-rafted debris beds to the respectively 635 Ma Marinoan and 582 Ma Gaskiers glaciations, and suggest that the oldest Dalradian glacial unit, the Port Askaig Formation, represents one of the c. 750–690 Ma Sturtian glacial episodes. These δ13C data and resulting correlations provide more robust constraints on the geological evolution of the Dalradian Supergroup than anything hitherto available and enhance its utility in helping refine understanding of Neoproterozoic Earth history.

Exploratory modeling: Extracting causality from complexity

On 22 May 2011 a massive tornado tore through Joplin, Mo., killing 158 people. With winds blowing faster than 200 miles per hour, the tornado was the most deadly in the United States since modern record keeping began in the 1950s.

Application of geochemistry to the stratigraphic correlation of Appin and Argyll Group carbonate rocks from the Dalradian of northeast Scotland

240 samples from Appin and Argyll Group carbonate rock units of the Dalradian of northeast Scotland have been analysed for 10 major oxides and 14 to 19 trace elements in order to classify and correlate them using geochemistry. The carbonate rocks are chiefly limestones, with some dolostones. Rocks with 0.38 > MgO/CaO > 0.03 are classed as dolomitic limestones; those with MgO/CaO > 0.38 are classed as dolostones. The data have approximately lognormal distributions at a high level of significance except for CaO, MgO and Sr. CaO and Sr have negatively skewed distributions. Median values for groups of sample analyses plotted on normalized multi-element variation diagrams and a nonlinear mapping (NLM) algorithm have been used to classify groups of samples. In the Appin Group, multi-element diagram patterns are distinctive for each unit, allowing correlations to be made. Though distinctive compositions do occur locally in Argyll Group carbonate rocks, correlation on a regional scale is not possible. The plot of NLM coordinates for suites of samples is consistent with the results from the spider diagrams; suites of Appin Group samples cluster closely whereas there is wide spread in samples from the Argyll Group. In several areas, most notably at Sandend Bay, the data have allowed reinterpretation of the stratigraphy. The data are consistent with other geological data in suggesting that crustal conditions were relatively stable during the deposition of the Appin Group, but not during the deposition of the Argyll Group.

Exploring the sensitivities of crenulate bay shorelines to wave climates using a new vector-based one-line model

We use a new exploratory model that simulates the evolution of sandy coastlines over decadal to centennial timescales to examine the behavior of crenulate-shaped bays forced by differing directional wave climates. The model represents the coastline as a vector in a Cartesian reference frame, and the shoreface evolves relative to its local orientation, allowing simulation of coasts with high planform-curvature. Shoreline change is driven by gradients in alongshore transport following newly developed algorithms that facilitate dealing with high planform-curvature coastlines. We simulated the evolution of bays from a straight coast between two fixed headlands with no external sediment inputs to an equilibrium condition (zero net alongshore sediment flux) under an ensemble of directional wave climate conditions. We find that planform bay relief increases with obliquity of the mean wave direction, and decreases with the spread of wave directions. Varying bay size over 2 orders of magnitude (0.1–16 km), the model predicts bay shape to be independent of bay size. The time taken for modeled bays to attain equilibrium was found to scale with the square of the distance between headlands, so that, all else being equal, small bays are likely to respond to and recover from perturbations more rapidly (over just a few years) compared to large bays (hundreds of years). Empirical expressions predicting bay shape may be misleading if used to predict their behavior over planning timescales.

How was the Iapetus Ocean infected with subduction

Because subduction in the Iapetus Ocean began only ∼35 m.y. after the end of rifting, spontaneous foundering of mature passive margins is an unlikely subduction-initiation mechanism. Subduction is more likely to have entered the Iapetus from the boundary with the external paleo-Pacific, similar to the incursion of the Scotia, Caribbean, and Gibraltar arcs into the modern Atlantic. The subduction zone probably became sinuous, entraining fragments of the Gondwanan margin along its complex sinistral southern boundary where oblique collision caused Monian-Penobscottian deformation. Following Taconian-Grampian collision of part of the subduction system with Laurentia, remaining parts of the Iapetus were progressively infected with subduction, leading to Silurian closure.

Scottish Landform Examples: The Cairngorms – A Pre-glacial Upland Granite Landscape

The Cairngorm massif in NE Scotland (Figure 1) is an excellent example of a preglacial upland landscape formed in granite. Glacial erosion in the mountains has been largely confined to valleys and corries (Rea, 1998) and so has acted to dissect a pre-existing upland (Figure 2). Intervening areas of the massif experienced negligible glacial erosion due to protective covers of cold-based ice (Sugden, 1968) and preserve a wide range of pre-glacial and non-glacial landforms and regolith. This assemblage is typical for many formerly glaciated upland and mountain areas around the world. The cliffs that sharply demarcate the edges of glacial valleys and corries allow the main pre-glacial landforms to be easily identified. The former shape of pre-glacial valleys and valley heads can then be reconstructed by extrapolation of contours to provide a model of the pre-glacial relief of the Cairngorms (Thomas et al., 2004). This relief model (Figure 3) provides a basis for understanding the development of the landscape over timescales of many millions of years, including the role of geology, weathering, fluvial erosion and, lately, glacial erosion in shaping the relief.