MORRISON TERRESTRIAL CARBONATES THEN AND NOW: WHERE WE WERE AND HOW MUCH HAS CHANGED?

The Morrison Formation consists of alluvial plain deposits that accumulated in a vast Western Interior lowland during the Late Jurassic as represented by fluvial, floodplain, eolian, and wetland-lacustrine rocks with minor evaporite and marine deposits. Within the alluvial complex, a major wetland-lacustrine carbonate succession is represented in the downstream distal lowlands.

In general, detailed regional studies of the carbonate deposits were lacking until recently. The carbonates were historically ignored because (1) major dinosaur quarries were not associated with limestones, so early bone hunters such as Cope and Marsh focused on the clastic rocks; because (2) the well-developed carbonate successions occur outside of the classic siliciclastic-dominated Colorado Plateau stratigraphic framework; and because (3) the carbonate deposits were not a laterally continuous, thick stratigraphic unit so many workers may have assumed these limestones were not terribly important.

With the inception of the Morrison Extinct Ecosystems Project in the mid-1990s, there was increased interest in understanding the carbonate deposits and the insights into paleoclimate and paleohydrology that these deposits offered. Today, we know that the wetland-lacustrine-palustrine carbonate intervals are well-developed in the undifferentiated Morrison Formation of east-central Colorado. Lacustrine intervals show evidence for surface water connections. Distal lacustrine units are composed of skeletal mudstone-wackestone, microbialites, and laminated (siliciclastic) mudstone. Marginal-lacustrine deposits include ooid and skeletal packstone-grainstone, siltstone, and sandstone with minor cross-stratification. Wetland deposits include charophyte-rich wackestone and green mudstone. Palustrine carbonates display evidence for frequent subaerial exposure and modification resulting from pedogenic activity, pseudo-microkarst, and meteoric diagenesis. The carbonate wetlands and lakes were well-oxygenated, reflecting holomictic to polymictic conditions. Stable isotopic compositions and sedimentologic evidence point toward an extensive complex of wetlands fed largely by groundwater discharge of regional aquifers through seepage and springs in an otherwise semi-arid climatic setting.