Jürgen Grötsch

Facies of sunken early cretaceous atoll reefs and their capping Late Albian drowning succession (Northwestern Pacific)

SummarySince first described in detail byHamilton (1956), the causes and timing of the drowning of several hundred guyots in the northwestern Pacific is a puzzling question. Thus, the northwestern Pacific is one of the key areas in deciphering the demise of flat-topped platforms throughout the earth’s history. Based on older paleontological data and the newly found shallow-water benthic foraminifera, the atoll reefs probably had a major period of vertical aggradation during the Barremian and the Aptian into the Late Albian depending on the stage of atoll development (type of guyot). New sedimentologic and stratigraphic data suggest a strong fall in sea level, leading to karstification and the formation of lowstand fringing reefs, prior to an even rapid rise of greater amplitude in the Late AlbianRotalipora appenninica zone ultimately causing drowning. After climatic relaxation, a sea level rise led to the final formation of small barrier reefs, rimming the top of many guyots in the Japanese Group, the Wake Group and the Mid-Pacific Mountains. They can be interpreted as “give-up” structures indicating a final shallow-water carbonate production on top of the atolls during drowning.The facies of the syn- and post-drowning sediments on the guyot tops are strikingly similar even when vast distances apart. This and the biostratigraphic data suggest a synchronous drowning of many seamounts investigated up to now.Biotic composition and facies of the final Albian reefs are very similar to Albian caprinid-dominated reefs in the Caribbean region, indicating comparable environmental controls.In the case of the northwestern Pacific guyots, the simultaneous demise of reefs could be due to a short-term cooling event in the Late Albian, connected with a strong regressive-transgressive cycle with an amplitude of about 180 m. This event is also known from the Tethys and the Atlantic. Climatic disturbances triggering short-term cooling and inducing a high amplitude regressive-transgressive sea level cycle, might be responsible not only for the Late Albian event, but also perhaps for other reef drownings throughout the earth’s history.

3D seismic geomorphology and sedimentology of the Chalk Group, southern Danish North Sea

Abstract: Classically, the North Sea Chalk is interpreted as having been deposited under quiet, homogeneous pelagic conditions with local redeposition in slumps and slides. Recent observations of highly discontinuous reflection patterns on 2D and 3D seismic reflection data from the NW European Chalk Group have led to a revision of some general ideas of chalk deposition, with the suggestion that long-lived, contour-parallel bottom currents exerted a primary influence on the development of intra-chalk channels, drifts and mounds. This study proposes an alternative explanation for the formation of selected intra-chalk seismic and stratal discontinuities, interpreting these as being caused by gravity-driven processes that developed in response to intense syndepositional tectonics. Submarine mass-transport systems identified in the study area include large-scale slumps, slides, debris flows and turbidites. The last occur in sinuous channel systems flanked by large master levees, with the channel fill exhibiting well-developed secondary banks and overbanks on the outer bends of the channel thalweg. This first documentation of channelized density-flow deposits in the North Sea Chalk has important consequences for the interpretation and prediction of redeposited chalk units, emphasizing at the same time the strength of detailed 3D seismic discontinuity detection for subsurface sedimentary-systems analysis.