Stonehenge—a unique Late Cretaceous phosphatic Chalk geology: implications for sea-level, climate and tectonics and impact on engineering and archaeology

Abstract

Ground investigations for the A303 Stonehenge Tunnels revealed a unique and complex Chalk geology including the presence of the thickest (>20m thick), and previously unknown phosphatic chalks in England, partly filling fault controlled erosional channels. The use of natural gamma-ray borehole logs to determine the presence and thickness of the phosphatic deposits is of particular value and combined with the lithostratigraphy, macrofossil and nannofossil biostratigraphy from cores has, for the first time, accurately constrained the Coniacian to Santonian age and the lenticular geometry of such deposits. Four phosphatic chalk events between 88.5-86.5 Ma are recognised associated with synsedimentary faulting. We suggest a causal link between tectonics, subsidence and channel-formation, phosphatisation events, pulses of oceanic upwelling on a frequency of about 0.5 million years to mantle-controlled plate tectonic episodes. The implications of this geology for construction of the A303 and the archaeology of the area are discussed. Key stratigraphical control Borehole R11 nannofossil analyses and age of the phosphatic chalks Borehole R11, because it contained the most horizons of phosphatic chalks, separated by 'normal' chalks, was chosen as the standard for further detailed stratigraphical analyses using

The genesis and evolution of karstic conduit systems in the Chalk

“…They are characterized by indurated, often iron stained horizons with fossil sponge remains. In some areas, hardgrounds are associated with phosphatic chalks (Mortimore et al, 2017;2019).…”

“…The flints are generally much smaller and less continuous compared to the underlying Lewes Nodular Chalk and Seaford Chalk formations, and tabular and sheet flints are not so well developed. Synsedimentary channels with hardgrounds and phosphatic chalks occur locally within the Newhaven Chalk, notably at Stonehenge (Mortimore et al, 2017). The overlying Culver Chalk is composed of up to 60 m of low to medium-density white chalks with some very strongly developed nodular and semi-tabular flints (Mortimore, 1986).…”

Comments on “Gale, A S., Norman Berry Peake, 1921-2010, a chalk revolutionary” [Proc. Geol. Assoc. 128 (2017), 829-839], and “Gale, A S & Lovell, B., The Cretaceous-Paleogene unconformity in England: Uplift and erosion related to the Iceland mantle plume” [Proc. Geol. Assoc. (2017) doi.org/10.1016/j.pgeola.2017.04.002]

“…Although many of the old chalk pits recorded in the 19 th and early 20 th centuries are now degraded or infilled, BGS surveying has yielded a huge amount of new biostratigraphical and lithological data from field brash, animal burrow spoil, and from relatively modern exposures, commonly at far greater densities than found by any of the earlier surveyors. For example, this approach was recently used to help map out the phosphatic channel around Stonehenge (Mortimore et al, 2017), in an area with very little historical locality data.…”