Laminations of the Oligocene Florissant Lake Deposits, Colorado

Abstract

Outcrops of the Oligocene Florissant Lake Beds occupy nearly 15 square miles in central Colorado. Faulting and erosion determine present outcrop distribution, and only the complex laminations which were deposited in the profundal zone of the lake are preserved. The lake deposits are composed primarily of volcanic debris, including pumice and ash, and of quantitatively minor biogenic components: diatomite and sapropel. The predominant type of lamination is the alternation of diatomite and sapropel which averages 1mm in thickness. Another type of alternation, with an average thickness of about 8 mm, consists of a graded tuff lamina overlain by one or more diatomite-sapropel couplets. A third type of lamination consists of inversely graded yellow pumice layers sporadically interbedded with diatomite-sapropel couplets; average thickness of the pumice layers is about 1.5 cm. Each diatomite-sapropel couplet represents 1year (varve) and is the normal sedimentary process. By analogy with modern periodicity of plankton production, the diatomite laminae are accumulations of spring diatom blooms. The sapropel represents late summer and early fall accumulations of other plankton. Graded pumice layers are associated with woody debris and were probably brought into the basin by floods. Most of the graded tuff laminae were deposited farther from shore, are more frequent and regularly occurring than pumice layers, and may be the result of storm redistribution of sediment from the margin of the lake. The nature and preservation of the Florissant laminations indicate a chemically stratified lake with a circulating, slightly alkaline upper layer and a stagnant, relatively acidic, oxygen-depleted lower layer.