LOFER CYCLOTHEMS REVISITED (LATE TRIASSIC, NORTHERN ALPS, AUSTRIA)

FISCHER's classic study (1964) of the Upper Triassic ''Lofer'' cycloth ems in the Dachstein Limestone of the Northern Calcareous Alps was sem inal to many studies of the Dachstein and to carbonate cycles globally . FISCHER's idealized cycle is deepening upward, ABC in his terminolog y, where member A is a surface or interval of subaerial exposure, B is tidal deposits, and C is shallow subtidal. Studies of the Dachstein f rom the elsewhere in the northern Alps have substantiated FISCHER's up ward-deepening ABC cycle, but GOLDHAMMER et al. (1990) and SATTERLEY ( 1996a) reinterpreted the type Lofer cycles as Shoaling upward. We meas ured 139 m of Dachstein Limestone incorporating 25 cycles at Steinerne s Meer, Austria, near FISCHER's most extensive section. In this sectio n we identified no A members. The section is punctuated by slightly re ddish horizons ('pink partings') that in some cases may reflect brief subaerial exposure, but generally appear to be pressure-solution zones that have concentrated iron oxides but lack a distinctive isotopic si gnature. B members are readily distinguished by fenestral porosity, st romatolitic lamination, partial dolomitization, intraclasts, or desicc ation cracks. They are relatively thin (5 to 155 cm; median thickness is 38 cm.) and in some cases laterally variable or discontinuous. C me mbers are characterized by molluscan wackestones and packstones with d iverse biota. C intervals are 25 cm to 26 m thick (median 4.1 m) and c omprise 91% of the interval measured. Pervasive bright-red internal se diment, which appears commonly within the B and C members, does not de rive from any interval observed within the measured section, but from sources, possibly paleosols, much higher in the section. It is spatial ly associated with near-vertical, ENE-trending (62 degrees) fractures filled with the sediment, brachiopods, and cement. Such fractures cut stratigraphic intervals as thick as 70 m without an exposed top or bas e. If ''pink partings'' were accepted as indicative of subaerial expos ure, three cyclothems (12%) would correspond to FISCHER's ideal upward -deepening cyclothem, seven cyclothems (29%) are shoaling-upward, four (17%) are symmetrical and the remaining 10 (42%) are incomplete with both deepening and shoaling components. If subaerial disconformities a re absent, the intervals are better described as BCBC rhythms than as true cycles. Our study is intended to stimulate new discussion of patt erns and origin of the Lofer cycles.