Patterns of diagenesis in the Sherwood Sandstone Group (Triassic), United Kingdom

Abstract

The Triassic Sherwood Sandstone Group comprises a complex of continental red beds deposited by a major fluvial system flowing dominantly down a northerly inclined palaeoslope. Sedimentation took place in several distinct, tectonically active basins with varying maximum burial depths, ranging from shallow (<1 km) to deep (>3 km). Despite proximal to distal variations in stream type, a distinct suite of early diagenetic events can be recognized throughout all the depositional basins, which is related to the depositional environment. These events are best preserved in those basins with shallow burial histories, and show many similarities to the processes recorded from modern red beds of the Sonoran Desert, Baja California, although a more advanced grade of diagenesis has been reached in the Sherwood Sandstone. On the margins of the Irish Sea Basin in Cumbria, where burial was shallow, these early diagenetic textures are well preserved. The detrital grains underwent changes aimed at reaching equilibrium with the near-surface chemical environment. Unstable silicates were dissolved or replaced and the released ions were capable of precipitating authigenic phases, typically mixed-layer illite-smectite, K-feldspar, non-ferroan carbonates and hematite. Lateral variations in the early diagnetic assemblages reflect chemical and spatiotemporal changes ofinterbasin depositional and diagenetic environments. Deeply buried equivalents in the Irish Sea reached a maximum burial depth in excess of 3 km towards the end of the Mesozoic. Superimposed on the early diagenetic fabric are a series of depth-related changes. In the absence of early cements, compaction reduced porosity to low levels. Mixed-layer illite-smectites were converted to highly crystalline illites with low Fe and Mg contents. Early framework-preserving non-ferroan carbonates were extensively dissolved, generating widespread secondary porosity. Late pore-filling carbonate cements comprise ferroan dolomites and ankerites with compositions up to Ca(Ca0·02Mg0·43Fe0·53Mn0·02). Following late Mesozoic burial, inversion of the Triassic basins resulted in the re-exposure of basin margin sequences and shallow burial basins. Modern groundwaters are typically very dilute, with essentially neutral pH, and are capable of dissolving carbonate, sulphate and halite cements. In the Wessex Basin, however, the Triassic sandstones of the South Devon aquifer have been extensively modified by the percolation of post-inversion acidic groundwaters. These low-pH waters have resulted in the in situ breakdown of feldspar to produce abundant authigenic kandite, widespread dissolution of carbonate cements and removal of early-formed iron oxide cements. In contrast, the concealed, deeply buried eastern margin of the Wessex Basin exhibits a diagenetic evolution resultant of deep burial and has no development of authigenic kandite.

Architectural analysis of a Triassic fluvial system: The Sherwood Sandstone of the East Midlands Shelf, UK

“…Channel fills with erosive bases are accompanied by lateral and mid-channel bar forms showing a mixture of lateral and downstream accretion. A braided river interpretation is concordant with observations by other authors for the lower part of the Sherwood Sandstone elsewhere in the UK and Ireland: Northwest England (Burley, 1984;Jones and Ambrose, 1994), East Irish Sea Basin (Cowan, 1993;1993b;Meadows and Beach 1993a), Southern…”

“…3), approximately 15 m above the presumed junction with the Lenton Sandstone Formation. The strata exposed have been described previously (Kitching et al, 1977;Burley, 1984;McKinley et al, 2011;McKinley et al, 2013), and summarised in Benton et al (2002), where the depositional setting of transverse barforms within a minor and major channels in a broadly northwards-draining major fluvial system was suggested.…”

Mica Alteration Reactions in Jurassic Reservoir Sandstones from the Haltenbanken Area, Offshore Norway

“…Simultaneous formation of kaolinite and K-feldspar by muscovite breakdown in a closed system (< 50~ was discussed on a thermodynamic basis by Bjorkum and Gjelsvik (1988) for Jurassic North Sea sandstones. Mica alteration to clay and other minerals has been described for several sandstone sequences from different parts of the North Sea and other basins of the worlds (see, e.g., Burley, 1984;Huggett, 1984;Morad, 1986a;Boles, 1987;Warren, 1987).…”

Clay Minerals in the MacAdams Sandstone, California: Implications for Substitution of H3O+ and H2O and Metastability of Illite

“…The normalized formulae have a slight excess of negative charge, probably corresponding to some K + loss during analysis (van der Pluijm et al, 1988), but the imbalance is small. The Kettleman North Dome I/S, or rectorite, has a composition similar to that ofauthigenic illite in general (e.g., Lanson and Champion, 1991 ;Jahren and Aagaard, 1992;Ireland et al, 1983;Macchi et al, 1990;Burley, 1984); there are around 1.5 /triAl and 1.4 interlayer cations per 12 [IV] + [VI], and the net negative charge is approximately 0.7-0.8 per half unit cell (Table 2). Such a composition does not correspond to an average ofillite-like and smectite-like layers, and it must be related to unique aspects of the ordered rectorite structure via simple ordering ofsmectite and illite layers (Ahn and Peacor, 1986;Jiang et al, 1990b).…”