ABSTRACT Widespread evaporitic sulphate horizons occur in the frequently dolomitic Mercia Mudstones (Upper Triassic) of the English Midlands. The mudstones were deposited on an extensive peneplain which had areas of upstanding minor relief (Charnwood Massif, Derbyshire—Pennines, London Platform). Horst and graben structures formed during the early and middle Triassic, controlled late Triassic sedimentation rates and peneplain slope directions. A stable isotope study (S,C,O) of the sulphates and dolomites of the Trent Formation indicates that during the deposition of the lower Fauld Member grabens such as the Need wood Basin in Staffordshire and areas marginal to a graben such as Newark in Nottinghamshire had a marine brine regime with minor continental input. Using constraints applied by sulphate concentrations of modern waters in arid environments, the ratio of volume of marine input to volume of continental input lay between 3:1 and 4:1. The horst area (East Leake, Nottinghamshire) was strongly influenced by continental brines which derived sulphate from the exposed Carboniferous Hathern Anhydrite Series. A predominantly continental brine regime existed across the whole area during deposition of the overlying Hawton Member. Periodic marine influxes gave rise to thin sulphate horizons whilst continental run‐off was occasionally sufficient to form temporary lakes. The Blue Anchor Formation (Rhaetian) was deposited in a lacustrine environment with waters of mixed marine and continental origin. Salinities were lower than those of the preceding Trent Formation brines.
More so than most of the past, the Archean is truly another country, with a geological record that is distinct from that of more recent epochs. The Archean covers a crucial 1500 Myr of Earth history, nearly three times the length of the entire Phanerozoic, from the earliest recorded rocks at its beginning to the growth of 60–70% of the continental crust by its close.
Summary A significant change in the nature of igneous activity responsible for continental growth apparently occurred at the Archaean-Proterozoic boundary. The sedimentary record is consistent with an Archaean upper crust dominated by the bimodal basic-felsic suite, with both units derived by melting at mantle depths. In contrast, the Proterozoic (and younger) upper continental crust is dominated by K-rich granites, derived by intracrustal melting. This major episodic event produced an upper crust enriched in incompatible elements (e.g. Rb, U) as attested inter alia by the increase in 87 Sr/ 86 Sr in marine carbonates in the early Proterozoic, and the abundance of early Proterozoic uranium deposits. This event seems to have been connected with the widespread initiation of subduction related calcalkaline volcanism, which, since that time, has been the principal contributor to continental growth. Although the change at the Archaean-Proterozoic boundary was the major episodic event in continental growth, minor episodes of cratonization occurred in the early Archaean, and some greenstone belt development lingered into the early Proterozoic. More recent episodes of continental growth (e.g. 1800 Ma event) appear to have been on a smaller scale. The sedimentary record is transparent to them, and they do not represent any fundamental change in the igneous processes responsible for continental growth.
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