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Abstract: Architecture of ancient forearc basin successions can be difficult to reconstruct because of the widespread syn-depositional and post-depositional deformations experienced by many forearc basin-fills. For this reason various techniques have been used for reconstructing forearc basin-infill geometry, including geochemical correlation. The Tyee Basin succession exposed in Coast Range of Western Oregon, USA, is an Eocene forearc-fill that includes genetically related non-marine, shallow marine and deepwater clastic deposits and is gently deformed. Reconstruction of the depositional geometry of the Tyee Basin succession from detailed outcrop and subsurface data reveals two distinct stages of development for this active basin-margin. These stages are characterized by two different basin-margin clinoform architectures and also by a pronounced change in the character of the associated deepwater deposits. During the initial stage, the basin-margin clinoforms are smaller (<250m clinoform height) and strongly progradational, with clinoform topset dominated by fluvial deposits. At this stage thick sand-rich unconfined turbidite beds accumulated on the slope segment of the clinoforms and extended out onto the basin-floor. Large scale slope conduits such as slope channels or canyons, are notably absent in this stage. The second stage is characterized by larger clinoform height (>500m), a greater degree of topset aggradation with repeated fluvio-deltaic cycles on the shelf, and well-organized, large turbidite channels on the slope. The turbidite channels supplied medium-grained sands to the extensive, stacked basin-floor fans. The first stage described above marks the early development of a shelf-slope prism on the Tyee continental margin, and has been interpreted by some earlier workers as an unique category of basin-margin architecture, termed as a ‘submarine ramp’. However, this was only the initial stage of development of the Tyee margin and it was followed by a period of basin-filling when repeated fluvial and shallow marine shelf-transit cycles fed well-organized turbidite channels on the slope as well as Tyee Basin floor fans. The large volume of sediment deposited during the initial stage, resulted from of the unique geometry of the Tyee Basin, as influenced by the presence of pre-existing topography on the accreted oceanic basement underlying the Tyee succession. [Copyright &y& Elsevier]

The Haystack Mountains Formation (Campanian, Mesaverde Group, US Western Interior Basin. Wyoming) contains a series of shallow-marine sandbodies, extending tells of kilometres out from a basin margin. The study succession (around 200 m thick) is composed of eight maior sandstone tongues (Bolten Ranch, O'Brien Spring, Seminoe 1-2-3-4, Hatfield 1 and 2 members), each partially encased within marine shale intervals. The Formation is 'sequential' at several scales. At the largest scale. the whole succession presents an aggradational to basinward-stepping stacking pattern of the sandstone tongues. At a lower level, each tongue (member) is characterized internally by two different types of lithosome: the first represents shoreface progradatlon with hummocky cross-strata passing up to swaley and trough cross-stratified sandstones. This lithosome is erosively truncated at its top in most cases, and has a general sheet-like geometry along strike, whereas down dip it displays a series of sharp-bounded clinothems. The latter sometimes indicate a downward as well as a basinward shift through time, as suggested by the occurrence of coarser and/or shallower facies at a lower level in the shoreface profile. The second type of lithosome is sheet- or wedge-like and sharply overlies the shoreface deposits. The lithosome consists of laterally widespread units of planar tabular to trough cross-bedded medium sandstones passing laterally (in a dip direction) into bioturbated sandstones. The lower part of this lithosome is progradational, becoming retrogradational into the overlying shales. The facies within the cross-bedded lithosome suggest a tidally dominated delta front to estuarine depositional setting. The two types of lithosome are nora related genetically. The erosion surface separating the two lithosomes is a sequence boundary separating forced-regressive (relative sea-level fall) shoreface deposits from lowstand to transgressive (early relative sea-level rise),... [ABSTRACT FROM AUTHOR]