Basin Research - Volume 3, Issue 2, 1991

Geometric analysis shows that the angle of migration of coastal sedimentary facies is a function of the relative sea‐level change and the thickness of sediment deposited or eroded. The angle of facies migration compared to the slopes on the sediment surface determines the degree of facies preservation and stratigraphic relationships to the surrounding facies.

Vertical facies successions generated by radial migration of environments show a great deal of variety because the sediment surface in both marine and non‐marine areas is concave‐up. Both regressive and transgressive sequences with non‐erosive marine‐nonmarine contacts can be generated. Transgression at a slightly lower angle can form a ravinement surface cut on non‐marine deposits with onlapping barrier sands or shallow marine deposits. Regression with relative sea‐level drop generates a minor erosion surface with baselapping isolated shoreline deposits. Disequilibrium conditions occur when sea level varies at a rate exceeding the ability of the system to supply or redistribute sediment, with resulting changes in surficial slopes.

Onlapping and downlapping stratal relationships across erosion surfaces result because of differences in slopes between marine and non‐marine environments. These discontinuities are generally less than one degree, but could possibly be recognized on high quality multichannel seismic lines. Most of these discontinuities are probably not regionally extensive enough to be regarded as sequence boundaries. Tectonic tilting or differential subsidence of strata during depositional hiatuses is necessary to generate true regional unconformities or sequence boundaries.

Where facies climb with respect to horizontal, erosion surfaces produced only by this migration may cut across lithostratigraphic units at higher angles, up to 3 or 4 degrees. Low‐angle erosion surfaces relevant to the scales of sequence stratigraphic studies may result only from facies migration, even during a period of relative sea‐level rise.

Compression of the thrust sheet underlying the central South Pyrenean Tremp‐Graus Foreland Basin led to weak folding of the overlying basin fill during deposition of Eocene sediments. From the distribution of sedimentary facies and the presence of these folds, it is interpreted that thrusting of deep‐seated competent units was accommodated by weak synsedimentary folding with a shorter wavelength at shallower levels. This led to differential subsidence at the surface but no unconformities are observed. The slower subsidence along the active anticlines locally influenced the distribution of sedimentary facies: lateral boundaries between different sedimentary facies are found to extend (sub)vertically up to more than 60 metres within narrow zones a few 100 m to 1.5 km wide. The growth folds thus led intermittently to the fixation of the position of facies boundaries, including a fixation of the coastline, over long periods (104 to 105 years). Sediment transport paths were also influenced by the slight folding of the surface.

The orientation of the weak ‘en echelon’ anticlines and of related facies boundaries agrees with the inferred compressional pattern during the Eocene.

Explanations for the regular occurrence of 50–60 m thick sedimentary cycles in terms of tectonic pulses or orbitally driven climatic changes and resulting pulses in sediment yield are discussed. For cycles of shorter length (10–15 m), with durations of the order of tens of thousands of years, it is inferred that these are due to regular climatic changes, probably related to orbital forcing, and resulting cyclic alternations of arid and wet periods. Such periodic changes of climate would have caused the intermittent waxing and waning of coarse‐grained sediment

A new interpretation of a comprehensive seismic‐ and well‐database has resulted in the subdivision of the Mesozoic into four, basin‐wide, seismo‐stratigraphic depositional megasequences in the Inner Moray Firth (IMF) basin. Regional mapping of the megasequences has led to the construction of a new model for Mesozoic‐Recent basin development in the IMF. It now appears that extensional tectonics was the main control on the basin's evolution during the Mesozoic. Structural geometries suggest that both the Triassic (Tr) and Rhaetian‐mid Oxfordian (Jl) megasequences were controlled by regional broad‐based subsidence associated with local extensional fault activity prior to the onset of renewed rifting in the IMF. In contrast, the late Oxfordian‐Ryazanian (Berriasian; J2) megasequence developed in response to active extension characterised by half‐graben development. Subsequent Early Cretaceous (Kl) deposition appears to have occurred during a further period of broad regional (thermal) subsidence. It is evident that strike‐slip movement on the Great Glen Fault played a negligible role in Mesozoic basin development and it appears only to have had a local control on structural styles during its reactivation in the Tertiary as it accommodated regional uplift and basin inversion.

Further subdivision of the J2 megasequence was possible using biostratigraphically‐controlled seismic reflector terminations and led to the definition of five regional seismo‐stratigraphic sequences (J2.1–2.5). Their geometric, thickness and sedimentary facies variations imply that the onlap‐defined sequence boundaries within the late Oxfordian‐Ryazanian (Berriasian; J2) megasequence were caused by syn‐sedimentary extensional tectonism in a fully marine domain, rather than by fluctuations in global sea‐level in a basin that was relatively quiescent tectonically. The new interpretation has particular significance in view of the fact that the Late Jurassic of the IMF was used by Exxon workers to construct part of their chart demonstrating relative changes of coastal onlap and global eustatic sea levels. As they considered that data from the area showed ‘no evidence that tectonics caused the unconformities’, the new interpretation casts doubt on the global applicability of the Late Jurassic section of Exxon's original sea‐level chart. Furthermore, the study demonstrates that reflector terminations within both tectonically active and/or fully marine sequences should be treated with extreme caution and not be used to define either periods of apparent low‐stand or coastal onlap. Indeed, their appearance may sometimes only represent relatively local, auto‐ and allo‐cyclic sedimentary processes such as submarine fan avulsion or channel switching, unrelated to changes in sea‐level. Finally, the work shows that care must be taken in the selection of seismic lines used to establish and illustrate the nature of depositional sequences and their geometries if pitfalls are to be avoided.

Shell‐Agip 35/13–1 well drilled 2445 m of Tertiary sediments in the Main Porcupine Basin situated offshore west of Ireland. Early Tertiary sediments and microfossils indicate a major cycle from deep‐sea to marginal marine and terrestrial palaeoenvironments returning to deep water. By means of seismic and lithostratigraphy and petrophysical logs, three deltaic cycles can be distinguished within this major cycle. The microfaunal zonation indicates that these cycles are of late Palaeocene, early Eocene and mid/late Eocene age and, therefore, correlate broadly with the Thanet Cycle, London Clay Cycle and the Bracklesham Cycles of the Anglo‐French type sections, although they are up to an order of magnitude thicker due to rapid basin subsidence.

Three major unconformities can be distinguished together with a disconformity that becomes an unconformity in the North Porcupine Basin. These surfaces are associated with both local and regional tectonic and igneous events. Detailed microfossil and lithological analyses across the major unconformities allows a reasonable matching with the global sea‐level curve and recognition of the major and medium sequence boundaries. Discrepancies during the late Eocene may relate to local faulting.

The pattern of sedimentation reflects the restriction of North Atlantic circulation and the tendency to euxinic bottom conditions during the early Palaeogene. In the middle Thanetian these conditions invaded the shelf, an event recorded elsewhere in NW Europe. Discontinuous seismic reflectors indicate ‘chaotic’ sedimentation connected with more vigorous circulation and erosion in the early Oligocene. This was followed by a change to parallel bedded contourites and drifts after the cutting of the early Miocene unconformity. The study reveals the complex interplay of eustatic and oceanographic change with local and regional tectonics in the development of the basin.

Book reviewed in this article:

Lacustrine Basin Exploration‐Case Studies and Modern Analogs. Barry J. Katz (ed.)

The Solid Earth. C.M.R. Fowler