LACUSTRINE FACIES TYPOLOGY IN THE TRIASSIC-JURASSIC RIFTS OF EASTERN NORTH AMERICA AND GREENLAND COMPARED TO THAT OF THE EOCENE GREEN RIVER FORMATION OF WYOMING

Olsen et al. (1) divided the "deeper water" lacustrine sequences of the Triassic-Jurassic rifts of central Pangea into five facies types. Crossing zonal climatic belts these are from the equator to about 30 degrees N Latitude: Richmond, Newark, Fundy, Fleming Fjord, and Kap Stewart type lacustrine sequences. Volumetrically significant potential source rocks for petroleum are found preferentially in the Richmond, parts of the Newark, and Kap Stewart type lacustrine sequences. These all record orbital forcing of water depth and hence climate, but in different ways.

We compare these rift sequences with the Eocene Green River Fm., famous for its "oil shales" (organic-rich marlstones) based on first hand observations of outcrop and core. In the Green River Fm. of Wyoming, the Laney Mb. consists largely of organic-rich laminated lacustrine marlstone and sandstone, which is at least locally cyclical. In the central parts of the basin, marlstones generally lack evidence of exposure or syndepositional evaporites. The Laney most closely resembles Richmond or Kap Stewart-type sequences, but differ from both in being primarily carbonate- rather than siliciclastic-dominated. The underlying Wilkins Peak Mb. is highly cyclical, but has much less laminated marlstone, more widespread signs of exposure (even in the basin center), and thick evaporite beds. The Wilkins Peak most closely resembles Newark-type lacustrine sequences, but differs in having much more evaporites.

In both of these sequences, climatic factors controlled the potential depth and evaporative concentration of the lake waters through such processes as precipitation and evaporation, tectonic factors controlled the realized maximum water depth and salinity through position of the outlet and catchment area and location and composition of basement rocks. Thus, actual lacustrine sequences do not fit into linear series of facies type or for that matter into a single plane of a phase diagram, e.g. (2), especially when comparing sequences with boundary conditions as different as those of the Triassic-Jurassic and Eocene.

1) Olsen et al. 2000. Epicontinental Triassic, v. 3, Zent. Geol. Palaont. VIII:1475. 2) Carroll, AR and Bohacs, KM. 1999. Geology 27:99.