The evolution of a thick shallow marine succession, the South Munster Basin, Ireland

Abstract

The relatively fine-grained Late Devonian to Early Carboniferous 'Cork Beds' succession of the South Munster Basin includes continuous sections of paralic facies that are over 1000 m thick and individual sandstone units over 300 m thick. However, the succession does not reflect prolonged phases when facies belts were stationary, but rather multiple stacking of small-scale, highfrequency sequences, each associated with pronounced migration of shorelines. What seems to have been unusual about the South Munster Basin succession was that the geographical positioning of these high-frequency sequences was fixed. This resulted from an unusual combination of tectonics, shelf hydrodynamics, sedimentation rates and the textural maturity of the sediment within the basin. Of these, tectonics was probably most critical, particularly the juxtaposition of rapid subsidence in basinal areas and a basin margin zone (to the north) that was sufficiently up-standing to pin the maximum extent of transgression during repeated highstands of sea-level, yet not so upstanding as to have diverted the major regional drainage system. The embayed palaeogeography of the area may also have been influential.

Is most hummocky cross‐stratification formed by large‐scale ripples?

“…, 2008; Myrow et al. , 2008; Quin, 2008). A further difficulty concerns explaining the presence of flat bedded sections up to 30 cm thick overlying HCS at bed tops (Dott & Bourgeois, 1982; Hunter & Clifton, 1982; Swift et al.…”

“…, 1977; Van de Meene et al. , 1996; Quin, 2008), isotropic HCS instead typically comprises lamination, which follows the external form of the bedform. Further, while the distinctive bundles associated with normal oscillatory ripples may exhibit switching cross‐lamination polarity reflecting the general back and forth migration of ripple crests under purely oscillatory flow (which in cross‐section might appear isotropic), this pattern would not be expected within the storm generated ripples which are proposed to form isotropic HCS.…”

“…Finally, there are also problems reconciling a large‐scale ripple model with the intimate relationship isotropic HCS exhibits with flat bedded sandstones generally (Figs 3 and 4). In particular, numerous cases where it is possible to observe lateral transitions from one structure to the other (Dott & Bourgeois, 1982; Craft & Bridge, 1987; Prave & Duke, 1990; Quin, 2001).…”

“…Transitional stratification (between flat beds and HCS) arguably is seen from the many flat bedded shallow marine sandstones which exhibit an undulating style (e.g. Kreisa, 1981; Arnott, 1993; Quin, 2008), and also by the fact that there are cases both where flat beds can be traced laterally into HCS (Fig. 4) and where flat bedded intervals are cross‐cut by isolated swales (Dott & Bourgeois, 1982; Cheel & Leckie, 1993; Quin, 2001).…”

“…Kreisa, 1981; Arnott, 1993; Quin, 2008), and also by the fact that there are cases both where flat beds can be traced laterally into HCS (Fig. 4) and where flat bedded intervals are cross‐cut by isolated swales (Dott & Bourgeois, 1982; Cheel & Leckie, 1993; Quin, 2001). It is perhaps also possible that some gutter casts are initiated from the same instabilities as isotropic HCS, as both structures are uniquely abundant in a similar range of storm‐influenced shelf settings (i.e.…”