RECOGNITION OF LATE JURASSIC CARBONATE WETLANDS AND LAKES FROM THE MORRISON FORMATION (COLORADO)

Wetland deposits are associated with an array of paleoecosystems and depositional regimes which make these deposits useful in interpreting paleohydrology and paleoclimate. Unfortunately, despite growing attention to and recognition of ancient wetland deposits, problems remain including: (1) significant confusion about “wetland” terminology and communication; (2) misidentification of wetland deposits as peritidal, estuarine, lacustrine, palustrine, floodplain, or paleosol; (3) simply relegating these deposits to the sedimentologic aquatic garbage can (i.e., “pond”); or (4) simply ignoring them.

Carbonate wetland and lacustrine deposits are recognized in the Morrison Formation of east-central Colorado, covering a stratigraphic interval equivalent to the “lower” Morrison but extending into the “upper” Morrison Formation. Sedimentologic, paleontologic, and isotopic data indicate that regional groundwater discharge maintained shallow, hydrologically open, well-oxygenated, perennial carbonate wetlands and lakes during the Late Jurassic despite the semi-arid climate. Wetland deposits include charophyte-rich wackestone and green mudstone. Lacustrine episodes, in which surface water input was significant, were times of carbonate and siliciclastic deposition in scarce deltaic, shoreline, and distal lacustrine units.

During Morrison Formation deposition, water that originated as precipitation in uplands to the west of the depositional basin infiltrated regional aquifers that underlay the basin. This regional groundwater system delivered water into the otherwise dry continental interior where it discharged to form a freshwater carbonate wetland and lacustrine succession in the distal reaches of the basin. Because groundwater was the major source of water with limited surface and meteoric contributions, the Morrison carbonates are considered to be primarily wetland in origin. During episodes of increased surface water input, lacustrine conditions developed.