Late Triassic sandstones from Quadrant 22 in the North Sea contain two generations of chlorite cement. Grain-coating chlorites consisting of curved, irregular crystal plates formed very early in diagenesis and are largely confined to marginal marine beds. Later diagenetic pore-filling chlorites, consisting of euhedral platy crystals, occur throughout the sequence in fluvial as well as marginal marine beds. Both generations of cement show similar XRD patterns; the 001 basal reflection of 14.0-14.06 A is broad and of low intensity, whereas the 002 reflection at 7.05 A is sharp and intense. Slight contraction of the 14 A basal spacing to 13.6 A occurs after heating to 600~ Electron microprobe analyses show the cements to be Fetich clinochlores with Fe/(Fe + Mg) ratios varying between 0.41 and 0-50. They are more Mgand Si-enriched than most other reported North Sea authigenic chlorites. The grain-coating chlorites have an average composition of (Fe3.06A13.93Mg3.32) [(Si6.76All.24)O20 ] (on)l 6. The pore-filling chlorites tend to show more variable compositions. The high Si values probably reflect both minor interstratification with smectite or vermiculite and contamination of the analyses by a closely associated illite/smectite clay. An origin related to alteration of a precursor swelling chlorite or corrensite is attractive on chemical and morphological grounds for the early grain-coating chlorites. However, direct precipitation from porewaters is the favoured origin for the later pore-filling chlorites, with the required ions being supplied by mineral decomposition reactions involving detrital silicate minerals and dolomite cements.Late Triassic sandstones of Skagerrak Formation affinity have been studied from three cored borehole sequences from Quadrant 22 in the Central Graben (Fig. 1). The sediments were deposited largely under a fluvial regime, but marginal marine facies occur toward the top of the succession. Typical examples of logged sections through these beds, together with diagnostic structures are shown in Fig. 2. While the sediments are dominantly fine-grained laminated sandstones, conglomeratic intraclast lags and mudstone beds occur at many horizons. The sediments contain abundant chlorite of both authigenic and detrital origin. Detrital flakes of chlorite generally occur in discrete laminae, often aligned along bedding planes, as matrix, or as reworked mudstone clasts. The authigenic chlorite occurs as thin isopachous coatings around detrital grains, as a pore-filling cement, and occasionally as a replacement of detrital feldspar and biotite grains. Facies analysis indicates that graincoating chlorite is largely confined to the marginal marine parts of the sequence, whereas the pore-filling chlorite occurs throughout the sequence.Diagenetic studies show the presence of a number of cements in addition to chlorite, including non-ferroan and ferroan dolomite, quartz overgrowths, a filamentous, ribbonshaped neoformed illite, feldspar overgrowths and local developments of anatase, and a variety...

Large trace fossils attributed to the ichnogenus Psilonichnus (Fürsich) have been identified in subtidal sediments of the Red Crag Formation (late Pliocene) of Walton-on-the-Naze, Essex, U.K. The traces are vertical or inclined, tapering cylindrical structures, occasionally branched, up to 2.3 m long. The structures are mud-rich relative to the surrounding sediment and their fills often show a spiralling grain fabric. The ichnogenus Psilonichnus has formerly been attributed to the burrowing activities of intertidal and supratidal crab species. The known fossil crab assemblage of the Red Crag contains no deep burrowing species. The traces are thus attributed to other crustaceans, possibly thalassinidean shrimps. The occurrence indicates that Psilonichnus may have a greater palaeoenvironmental range than previously realized.

A petrographical investigation of the sandstones of the Brent Group (Middle Jurassic) from the Murchison field has produced evidence for the existence of two transgressive events within the sequence, one occurring immediately above the Etive Fm., the other within the upper part of the Ness Fm. These transgressions have not been previously recorded in Murchison and their discovery demands that some modification be made to the existing depositional model for the Brent Group of this area. A volcanic ash horizon has been found in a similar stratigraphic position to that recorded from Statfjord, and if this, and the two transgressive horizons noted above, are of more than local extent they could prove to be useful stratigraphic markers. Although the diagenesis of the Brent Group sandstones in Murchison differs little from that recorded elsewhere in the Brent province, the interpretation of the timing of the development of certain phases differs somewhat from those of other authors. In particular, kaolinite development appears to have taken place soon after burial in response to periods of emergence and weathering during Brent Group deposition, rather than through groundwater circulation during the Kimmerian tectonic phase.

Grain-coating chlorite cements commonly occur within sandstones of late Middle and Upper Miocene age deposited in the North Sumatra back-arc basin. Chlorites from the Lower Keutapang Member contain Ca (maximum 0.75 wt% oxide) and show textural evidence for direct precipitation on grains. However, crystals are subhedral, showing curved faces and often ragged edges, and show a tendency to merge together. In overlying beds of the Upper Keutapang Member, grain-coating chlorite-smectite (20% smectite) cements display an identical morphology but are more siliceous, have a lower octahedral occupancy and contain higher total (Na + Ca + K). It is proposed that chlorite cements in the Keutapang Formation originated as smectite-rich cement rims whose initial precipitation was related to the breakdown of volcanic detritus in the sediments. Transformation to chlorite occurred subsequently during burial, facilitated by a high geothermal gradient in the back-arc basin.