Geophysics, Geodynamics and Tectonics
Measuring the Pulse of Mantle Plumes with the Sedimentary Record
Sandstones have been an obsession of mine for over 40 years. Both Nick McCave (now Emeritus Professor here at Cambridge) and I came under the great academic and practical influence of Harold Reading at Oxford in the early Sixties. We also both went off to the Ivy League to work on sandstones for our PhDs. At Harvard my work on the Eocene Tyee Formation of the Oregon Coast Range was supervised by the late Raymond Siever who liked to see knowledge applied.
Since then, through lengthy spells as first a University Lecturer in Geology at Edinburgh, then as Chief Sedimentologist and Exploration Manager with BP (and also as Chairman of a couple of the smallest companies in Scotland and England) I have never shaken off that early belief that sandstones really are useful in all sorts of ways.
In 1995 Nicky White and I formed the hypothesis that it is possible to measure the pulse of mantle plumes using the sedimentary record: we proposed (1997) that the well documented discrete episodes of sand deposition in the Paleogene of the North Sea reflect pulses in the early Iceland plume. Mantle plumes have notable and measurable effects on regional sea-levels. There is a transient thermal effect that may be seen as a temporary departure from the main trend on plots of basin subsidence: Steve Jones and others (2001) provide an example of this from the Paleogene of Porcupine Basin. Max Shaw Champion and others (2008) use 3D seismic and well data to quantify transient mantle convective uplift of the seabed on the west and east flanks of 55 million-year-old Scotland. This has enabled John Rudge and others (2008) to generate a model of the thermal pulse in the early Icelandic plume that may be held responsible for the sequential uplift as it travelled from west to east in the convecting asthenosphere.
There is also evidence of more permanent uplift that results from magmatic underplating associated with the presence of the plume. Study of the sedimentary record of the repeated marine regressions and transgressions in the Paleogene rocks around Scotland indicates that this underplating takes place episodically, if not periodically, at intervals of the order of a million years or more. It may be regarded as a special case of the more general Rudge model. John Maclennan and I suggest (2002) that, in the case of underplating, the episodes of subsidence that follow shortly after the pulses of surface uplift, may be explained by invoking rapid crystallisation of the underplate.
This quantifiable process of pulsing hotspots provides an explanation for rapid regional changes in sea level. The model does not invoke global effects and is therefore an alternative to the classic Exxon model. Our hypothesis may be tested with particular confidence on sediments formed during the long periods of geological time when Earth had no significant ice-sheets and hence no glacial-eustatic control of sea-level.
Such tests have been carried out in the North Atlantic, in Africa, Australia and India. Current work is focused on Africa and the North Atlantic.