Controls on the Sedimentological Heterogeneity of Visean Carbonate Platforms in England and Wales

Carbonate rocks (limestones) can record the depositional environments of prehistoric seas. The
position of the sea in space and time controls the types of organisms which live within it, how they are
distributed, and how they build into a body of sediment with topographic relief, known as a carbonate
platform. This is because limestones are built from the calcium-carbonate skeletons of marine organisms
which are highly sensitive to light, nutrition, temperature, and salinity (the "carbonate factory"). During
the Lower Carboniferous Period (362.5 million years ago), tectonic plates reassembled by continental drift
to build supercontinent Pangaea. This led the UK to be positioned at around 14 degrees south of the equator,
comparable to the present-day position of Brazil or Madagascar. The north of the UK was positioned
under the sea, giving the potential for limestones to be deposited.
The carbonate platforms were deposited in two stages over approximately 11.1 million years: the
Asbian (oldest) and the Brigantian (youngest). Asbian rocks (strata) were laid down during a greenhouse
climate, defined as a world with no ice at the poles. This meant that sea-level did not fluctuate much
because sea water was not periodically frozen into ice caps. A large ice mass (the Gondwanan ice sheet)
began to develop at the south pole during Asbian, and an Icehouse climate had established by the
Brigantian. As a result, sea-level fluctuated considerably by the Brigantian, in terms of frequency and
amplitude, and seasons (e.g. periodic wet/dry seasons) were amplified. This meant that the carbonate
factory existed under more stressed conditions and was influenced by systems on land (terrestrial), such
as rivers and deltas.
As well as a significant change in climate, the carbonate platforms were also subjected to active
tectonic movement. During the Asbian, the rocks were being pulled apart, to form a series of faultbounded
blocks (half-grabens) which are 1000s of km2 in size, which have topographic highs. The best
conditions for organisms to thrive was upon the highest topography, where light and temperature were
optimal. By the end-Brigantian, the strata had been pulled apart enough for thermal subsidence to occur,
whereby the cooling mantle (beneath the crust) contracted to become part of the lithosphere (crust and
upper mantle) which is denser than the surrounding material and consequently subsided and sea-level
increased around the carbonate platform.