Some of the sediments of the Green River Formation are very thinly laminated. These laminae were originally interpreted as varves, which are annually deposited layers, usually forming a dark/light couplet per year reflecting seasonal changes in organic productivity or sedimentation rates (Bradley, 1929). However, the varve interpretation presents some problems. Many Green River fish are found well preserved between mm-thick varves. Such dead fish would have to lie undisturbed for several years to be gradually covered with a veneer of very thin laminae. The excellent preservation of many Green River fish points to much more rapid burial than implied by the annual varves interpretation. Hellawell and Orr (2012) proposed that the laminae formed due to the trapping of sediments by microbial mats covering the lake bottom. These microbial mats may have embedded dead fish preventing its disarticulation. However, Rogers et al. (2024) found no convincing evidence supporting an original presence of microbial mats on the bottom sediments.

Another problem for the varve interpretation is that the number of laminae changes laterally in an interval of sediment representing the same amount of time, with more laminae closer to the shore than in the basin center. Buchheim (1994) concluded that the laminae should not be considered annual varves but reflect pulses of fluvial input into the lake and subsequent carbonate precipitation. More recent publications acknowledge that lamina deposition may have been much more frequent than twice per year (e.g., Grande 2013, p. 12).

References:

Bradley, W.H., 1929. The varves and climate of the Green River epoch. United States Geological Survey. No. 158-E.

Buchheim, H.P., 1994. Paleoenvironments, lithofacies and varves of the Fossil Butte Member of the Eocene Green River Formation, southwestern Wyoming. Contributions to Geology, University of Wyoming, 30(1), pp. 3-14.

Grande, L., 2013. The lost world of Fossil Lake: Snapshots from deep time. University of Chicago Press. p. 425.

Hellawell, J. and Orr, P.J., 2012. Deciphering taphonomic processes in the Eocene Green River formation of Wyoming. Palaeobiodiversity and Palaeoenvironments, 92, pp. 353-365. doi:10.1007/s12549-012-0092-6.

Rogers, G.A., Pittinger, D.R., Simpson, E.L. and Friehauf, K., 2024. Microscale analysis of the fish Knightia eocaena taphonomy: Implication of a preserved microbial community. Palaeobiodiversity and Palaeoenvironments, pp. 1-16. https://doi.org/10.1007/s12549-024-00621-6