New biostratigraphical analysis of the Humber Group sediments from the UK North Sea Central Graben, using integrated palynological and micropalaeontological zonation schemes, allows substage resolution and basin-wide discrimination of subunits of the Heather Formation/Kimmeridge Clay Formation, and associated sandstones. The integrated use of both zonation schemes is particularly important because thermally mature Humber Group sediments buried below 12 000 ft in the Central Graben cannot be dated consistently to substage level using palynology alone. Poor data and the undiagnostic log character of the mudstones and shallow marine sandstones have resulted in a simple lithological scheme being widely applied, with all sandstones assigned to the Fulmar Formation and mudstones to the Heather Formation and the Kimmeridge Clay Formation. Previous generalized usage of the Fulmar Formation for all Upper Jurassic shallow marine sandstones is inappropriate. The Upper Oxfordian Puffin Formation is present in the northern and western Central Graben. The largely Kimmeridgian Fulmar Formation is restricted to the south and east, and the coeval Ula Formation flanks the Jaeren High. These three units are all interpreted as syn-rift sandstones, locally sourced from uplifted and eroded fault block crests within and marginal to the Central Graben. Latest Kimmeridgian or younger shallow marine sandstones are largely confined to platform areas outside the graben, and can be related to regressive phases.

The identification and correlation of genetic sequences has significantly enhanced our understanding of the Late Jurassic stratigraphy in the Central North Sea. The integration of genetic sequences with an appreciation of the basinal mudrock stratigraphy, and the identification of depositional sequence boundaries provides a powerful tool to understand and potentially predict the distribution of subtle stratigraphic turbidite plays. The largely syn-rift Late Oxfordian-Kimmeridgian was deposited during a low frequency, second-order sea-level rise, culminating in the eudoxus condensed section. Deposition was dominated by shallow marine shelfal sandstones and offshore mudrocks. Sandy turbidites are rare. Whilst the requisite depositional slopes and deep water areas are likely to have existed, the effects of high frequency relative sea-level falls are likely to have been suppressed. As the second-order sea-level rise accelerated towards the eudoxus condensed section, the supply of sand into the basin was reduced such that the potential for turbidite deposits was minimal. The largely post-rift Kimmeridgian-Volgian section is dominated by basinal mudrocks. However, regionally correlatable ‘hot’ and ‘cool’ units provide information on sediment flux into the basin which may be linked to relative sea-level changes on the basin margin, with cool units interpreted to be the product of enhanced depositional rates during lowstand. Turbidites commonly occur within the ‘cool’ mudrock units. A Middle Volgian ‘cool’ unit is likely to represent a second-order sea-level fall, within which are composite turbidite packages which are considered to be the product of higher frequency lowstands (lowstand sequence set).

No abstract

A partir del análisis de datos gravimétricos del oeste de Texas y norte de México se delinean dos áreas mayores caracterizadas por prominentes altos gravimétricos lineales. Una de las áreas corresponde a la Plataforma Central de la Cuenca Pérmica y la otra corresponde al Levantamiento Tascotal, de la región de Big Bend. La orientación en ambos casos es oblicua al margen continental inferido para el Paleozoico temprano. En la plataforma de la Cuenca Central se ha documentado la presencia de un cuerpo máfico. En el Levantamiento Tascotal, la respuesta gravimétrica es compatible con la presencia de un cuerpo máfico. Nosotros interpretamos las dos áreas en términos de reactivación de núcleos máficos de ‘rifts’ fallidos o incipientes desarrollados durante el establecimiento del margen continental sur de Norteamérica en el Precámbrico tardío - Paleozoico temprano. Esta reactivación durante la actividad orogénica compresional del Paleozoico tardío dominó la evolución estructural de la región y debe haber influenciado grandemente la sedimentación y los patrones de circulación oceánica en la región durante el Paleozoico tardío.