(^94) H. A. ARMSTRONG & A. W. OWEN
Fig. 8. Plots of maximum recorded generic diversity for Laurentia (a. b). Avalonia (c, d) and Baltica (e, f) for
biofacies through time. Ocean temperature for Laurentia is known directly from stable oxygen isotope data
(Brenchley et al. 1995b), Global cooling was initiated in the early Ashgill (Armstrong & Coe 1997) with the
glacial maximum in the late Ashgill (Hirnantian). Temperatures for Avalonia and Baltica are inferred from
palaeogeographical reconstruction and the drift northwards of Avalonia into the cooling tropics in the mid-
Ashgill (Fig. 9). Abbreviations: LL-C, Llanvirn-Caradoc: vel. vclicuspis Biozone: ord. ordovicicus Biozone:
celloni, celloni Biozone.
African shelf, associated with the cold Benguela
Current, is illustrated in Figure 10. Here,
upwelling is driven by persistent offshore winds
that skim off warmer surface water allowing
cold, nutrient-rich, oxygen-poor subsurface
water to ascend from intermediate depth
(Demaison & Moore 1980; Fig. 10). In compari-
son with this modern analogue we propose that
the oceanic biofacies identified in our analysis
represent water-mass-restricted faunas and the
distribution of OB3 biofacies reflects an area of
cold, nutrient-rich, oxygen-poor water (Fig. 10).
The differential vertical movement of OB3
relative to OB1 and OB2 (Fig. 7) therefore prob-
ably reflects upwelling adjacent to the Avalonian
and Baltic margins of the Iapetus Ocean.
The presence of vigorous upwelling would
have a profound effect on the sedimentology of
outer shelf areas, with the deposition of black
shales and potentially phosphate enrichment of
the sediment. The high percentage of phosphate
in limestone of the late Caradoc Nod Glas
