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Upper Cretaceous and lower Tertiary rocks exposed in the Raton basin, New Mexico and Colorado, contain a thin, indium-rich, kaolinitic clay bed that marks the palynologically defined Cretaceous-Tertiary boundary. The clay bed is presumed to have been deposited as the result of a catastrophic event, possibly the impact of an asteroid, at the end of the Cretaceous; it is preserved in sediments laid down in the quiet waters of ponds and coal-forming swamps. Occurring in a conformable sequence of nonmarine rocks, this extraordinary marker bed enables observation of depositional conditions of an instant of geologic time throughout a wide area in the basin. Investigations made at various sites where the boundary clay is present indicate that component elements of a dynamically aggrading fluvial system existed at the close of the Cretaceous. This depositional system was characterized by meandering rivers, broad floodplains, and levees. The levees were breached locally by crevasse splays that periodically invaded the floodplains and associated poorly drained and well-drained swamps.

At the end of 1980, seven complete cores were recovered from a 30-m (100-ft) interval in the Raton Formation at York Canyon, New Mexico. The interval cored spans the palynologically defined Cretaceous-Tertiary boundary, which is marked by a distinctive noble metal–bearing claystone in the Raton basin. Azimuthal orientation of the cores can be recovered both from the average directions of the most stable components of the remanent magnetization, with a root mean square error of 28°, and from the average direction of secondary components of magnetization removed by thermal and alternating field demagnetization, with a root mean square error of 33°. The natural remanent magnetization of about 95 percent of the core is dominated by a secondary normal polarity component. Polarity of the characteristic magnetization of each core, interpreted from 12 to 14 samples per core run, is reversed. No evidence of normal polarity characteristic magnetization was found in the 30-m (100-ft) interval sampled. The characteristic magnetization probably is a depositional remanent magnetization acquired during chron 29r. The noble metal–bearing boundary claystone in the Raton basin is interpreted to be part of a synchronous global deposit laid down at the end of the Cretaceous period.

Abstract The Raton basin is bounded on the west by the ancestral San Luis highlands. The highlands were probably uplifted along a highangle reverse fault zone marginal to the basin. Coarseningupward megacycles and types of facies sequences and facies associations within the Upper Cretaceous Vermejo Formation and Upper Cretaceous and Tertiary Raton and Poison Canyon Formations indicate that both allocyclic and autocyclic mechanisms influenced alluvial sedimentation in the Raton basin. The megacycles comprise a coalrich, fine detritusdominated facies and an overlying coalpoor, sanddominated facies. The coalrich, fine detritusdominated facies was deposited by braided, meandering, and anastomosed fluvial systems during periods of increased basin subsidence associated with a stable source area. The sanddominated facies was deposited by alluvial fans and by braided and highbedload meandering fluvial systems during periods of tectonic uplift in the source area and reduced basin subsidence. The basin alluvial paleoarchitecture (spatial arrangement, patterns, interconnection, distribution, and evolution of the fluvial systems) was mainly controlled by allocyclic mechanisms of uplift in the source area and subsidence in the basin. Climatic change and the presence of vegetation played only minor roles in controlling the alluvial paleoarchitecture of the Raton basin. In this alluvial environment, basin subsidence (an allocyclic mechanism) and fluvial avulsion and abandonment (autocyclic mechanisms) served as the major controlling factors in the accumulation of thick coal beds.