ALBERT

Description: Well-developed Campanian to Maestrichtian pelagic cyclic sediments were recovered from Hole 762C on the Exmouth Plateau, off northwest Australia, during Ocean Drilling Program Leg 122. The cycles consist of nannofossil chalk (light beds) and clayey nannofossil chalk (dark beds). Both light and dark beds are strongly to moderately bioturbated, alternate on a decimeter scale, and exhibit gradual boundaries. Bioturbation introduces materials from a bed of one color into an underlying bed of another color, indicating that diagenesis is not responsible for the cyclicity. Differences in composition between the light and dark beds, revealed by calcium carbonate measurement and X-ray diffraction analysis, together with trace fossil evidence, indicate that the cycles in the sediments are a depositional feature. Diagenetic processes may have intensified the appearance of the cycles. Spectral analysis was applied to the upper Campanian to lower Maestrichtian cyclic sediments to examine the regularity of the cycles. Power spectra were calculated from time series using Walsh spectral analysis. The most predominant wavelengths of the color cycles are 34-41 cm and 71-84 cm. With an average sedimentation rate of 1.82 cm/k.y. in this interval, we found the time durations of the cycles to be around 41 k.y. and 21 k.y., respectively, comparable to the obliquity and precession periods of the Earth's rotation, which strongly suggests an orbital origin for the cycles. On the basis of sedimentological evidence and plate tectonic reconstruction, we propose the following mechanism for the formation of the cyclic sediments from Hole 762C. During the Late Cretaceous, when there was no large-scale continental glaciation, the cyclic variations in insolation, in response to cyclic orbital changes, controlled the alternation of two prevailing climates in the area. During the wetter, equable, and warmer climatic phases under high insolation, more clay minerals and other terrestrial materials were produced on land and supplied by higher runoff to a low bioproductivity ocean, and the dark clayey beds were deposited. During the drier and colder climatic phases under low insolation, fewer clay minerals were produced and put into the ocean, where bioproductivity was increased and the light beds were deposited.

Description: Analysis of sand-body distribution reveals that fluvial channel sands in the Upper Cretaceous lower Williams Fork Formation in the central Piceance Basin, Colorado, USA, are not randomly distributed but are predictable in their spatial and stratigraphic position. Sand bodies are organized in channel-belt clusters that are compensationally stacked on different temporal scales. While channel clustering occurred over a shorter period (channel and channel-belt scale 10s ky to 100s ky), compensational stacking occurred over a much longer time scale (channel cluster belts, [~] 400 ky). The lower Williams Fork Formation consists of three distinct intervals of clustered channel belts, each [~] 400 ft ([~] 122 m) thick, topped by a thin coal layer. The primary control on the cluster formation is autogenic channel avulsion during an overall aggradational phase. Short-lived changes in in basin accommodation caused by either changes in tectonic subsidence or high-frequency eustatic changes due to Milankovitch-band orbital forcing at the end of each cluster interval result in a distinct increase in channel thickness and coal formation towards the end of each cycle. Coals at the end of each sub-cluster span the entire study area, but their overall thickness is greater in the inter-cluster, floodplain-prone areas. The resulting differential early coal compaction is greatest in the inter-cluster areas and has a significant control on the subsequent cluster-belt position overlying the coal. Channel sands overlying the coals are concentrated in the previously low net/gross floodplain-prone inter-cluster areas resulting in compensationally stacked cluster belts. On the longest time scale ([~] 1 to 1.5 My), which encompasses the entire lower Williams Fork, changes in channel geometries and sand-body thickness seem to be controlled mainly by long-term changes in eustasy, and autogenic processes are of lesser importance.

Description: The Sunnyside Member of the Upper Cretaceous Blackhawk Formation in the Book Cliffs of eastern Utah provides an ideal opportunity to investigate high-resolution sequence-stratigraphic correlation between shallow-marine and terrestrial strata in an area of outstanding outcrop exposure. The thick, laterally extensive coal seam that caps the Sunnyside Member is critical for correlating between its shallow-marine and terrestrial components. Petrographic analysis of 281 samples obtained from 7 vertical sections spanning more than 30 km (18 mi) of depositional dip enabled us to recognize a series of transgressive-regressive coal facies trends in the seam. On this basis, we were able to identify a high-resolution record of accommodation change throughout the deposition of the coal, as well as a series of key sequence-stratigraphic surfaces. The stratigraphic relationships between the coal and the siliciclastic components of the Sunnyside Member enable us to correlate this record with that identified in the time-equivalent shallow-marine strata and to demonstrate that the coal spans the formation of two marine parasequences and two high-frequency, fourth-order sequence boundaries. This study has important implications for improving the understanding of sequence-stratigraphic expression in terrestrial strata and for correlating between marine and terrestrial records of base-level change. It may also have implications for improving the predictability of vertical and lateral variations in coal composition for mining and coalbed methane projects. Roy Davies is a postdoctoral researcher for the Center for Integrated Petroleum Research at the University of Bergen in Norway. His research interests include coal petrology, sequence stratigraphy, reservoir modeling, and the integration of electromagnetic and seismic data. He holds a B.Sc. degree and a Ph.D. in geology from the University of Liverpool, United Kingdom. John Howell is a professor at the Center for Integrated Petroleum Research at the University of Bergen in Norway. His research interests include reservoir modeling, sequence stratigraphy, and clastic sedimentology. He holds a B.Sc. degree in geology from the University of Cardiff and a Ph.D. in geology from the University of Birmingham, United Kingdom. Ron Boyd is an associate professor at the University of Newcastle in Australia, where his research interests include marine geoscience, sedimentology, and stratigraphy. He holds a B.Sc. degree and a Ph.D. from the University of Sydney and has previously taught and researched at Louisiana, Dalhousie, and New Hampshire universities, the Royal Australian Navy Research Labs, Geomar Institute Kiel, and PanCanadian Petroleum Calgary. Stephen Flint holds a personal chair in stratigraphy and petroleum geology at the University of Liverpool in the United Kingdom. After obtaining his Ph.D. from the University of Leeds in 1985, he spent 6 years working for Shell Research in the Netherlands before returning to the United Kingdom to set up the Stratigraphy Group at the University of Liverpool in 1991. Claus Diessel is an emeritus professor at the University of Newcastle in Australia and has extensive research interests in many aspects of coal petrology, as well as its application to sequence stratigraphy. He holds a Dipl. Geol. and Dr. Ret. Nat. from the University of Berlin, Germany.