ABSTRACT The late Eocene Florissant Formation in central Colorado is a rich and diverse continental Lagerstätte yielding well-preserved fossil assemblages from lacustrine and fluvial facies. This investigation focused on the lacustrine facies at Clare’s Quarry and used biotic and abiotic evidence to characterize aspects of the lake and processes that resulted in the accumulation and preservation of the host rock and its fossils. Autecology of modern analogs representing the fossil diatom taxa was used to augment sedimentary data in characterizing the lake, propose peripheral habitats within the catchment area, and suggest a terrestrial source for mudstone units. The sedimentary and stratigraphic record at the study site reveals a lake with sufficient depth to allow bottom waters to remain isolated and anoxic for long periods. Sediments that accumulated in the lake produced distinct lacustrine lithofacies that are interpreted as representing at least three modes of origin: stable lake, pyroclastic, and mud turbidite sedimentation. Slow, suspension settling of fine clays and volcanic ash into a moderately deep, stable lake resulted in laminated shales. These laminated shales contain frustules of diatoms from planktic and benthic lake habitats; diatoms transported into the lake from streams and wetlands; fish, mollusks, ostracods, and insects; and plants from marginal and upslope environments. Intermittent volcanic eruptions produced air-fall ash and granular tuff that accumulated as interbeds within the lake shales. Periods of stable lake sedimentation were frequently interrupted by rapid influxes of suspended fine clays, perhaps as mud-dominated turbidites that prograded into the lake at intervals of high runoff triggered by climatic, volcanic, or tectonic events.

Abstract The Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2) is the last major OAE of the Mesozoic and probably the best studied. In marine rocks around the Gulf of Mexico it is associated with a variety of different environments, from well-oxygenated carbonate platforms to anoxic, organic-rich outer shelf environments and un-studied basinal muds. This paper reviews the current level of knowledge about the geographic distribution and stratigraphic expression of OAE2 in the Gulf of Mexico in order to synthesize this disparate data and attempt to draw some conclusions about regional oceanography during this critical interval of the Cretaceous. A large number of localities with varying local redox states have been tied to OAE2, including the Valles-San Luis Potosí and Guerrero-Morelos platforms in southern Mexico, deep shelf sites in northern Mexico, the well-studied outcrops and cores of west Texas on the Comanche platform, cores and wells along the Barremanian-Albian shelf margin of south Texas, geophysical data in the East Texas basin, cores in the Marine Tuscaloosa Formation of Louisiana, Alabama, and Mississippi, and deep wells in the deep water Gulf of Mexico. The distribution of anoxic sediments at these sites during OAE2 appears to be determined by water depth. Shallow sites, like the Mexican carbonate platforms and the Comanche platform of Texas, are oxygenated during the event. Deeper shelf sites, like the south Texas Rio Grande submarine plateau and the noncarbonate platform parts of the Mexican shelf, are anoxic and enriched in organic carbon; it seems likely that this trend continues across the rest of the Cretaceous Gulf shelf, although data is sparse. Whether this oxygen minimum zone only impacts the deeper parts of the shelf or extends all the way to the basin floor is the most significant outstanding question about OAE2 in the Gulf of Mexico.