TRACE FOSSILS AND RHIZOLITHS AS A TOOL FOR SEDIMENTOLOGICAL AND PALEOENVIRONMENTAL ANALYSIS OF ANCIENT CONTINENTAL EVAPORITE SUCCESSIONS

Recognition of evaporite formations from continental Tertiary basins o f Spain provides evidence that trace fossils (including rhizoliths) ca n be abundant in some saline lake systems and their study helps in pal aeoenvironmental interpretation of ancient continental evaporite seque nces. Six main types of trace fossils have been distinguished and incl ude: (1) networks of small rhizoliths; (2) large rhizoliths; (3) tangl e-patterned small burrows; (4) isolated large burrows; (5) L-shaped tr aces; and (6) vertebrate tracks. Rhizoliths were related to both margi nal areas of hypersaline lakes and lakes of moderately high saline wat ers. In these settings, pedoturbation resulted from colonization by gr asses and bushes of distinct lake subenvironments. The activity of bur rowing invertebrate faunas was especially intense in lakes of moderate ly concentrated brines from which gypsum was the main evaporite minera l deposited. A specific gypsum lithofacies ('bioturbated gypsum deposi ts') forming thick, massive beds has a widespread occurrence in many o f the basins, Tangle-patterned small burrows and minor isolated large burrows constitute the typical trace fossil types within the gypsum. T he traces are interpreted as having been caused by burrowing insect la rvae, probably chironomids, coleopterans and annelids, The behaviour o f these organisms in recent lake environments yields information about the salinity range of lake waters from which gypsum precipitated, Con centration values averaging 100-150 g/l may be thus deduced though som e organisms involved in the formation of the traces can tolerate highe r salinities. The combined analysis of lithofacies and trace fossils f rom the lacustrine evaporite sequences contributes to the study of dis tinct saline lake subenvironments as well as changes in the sedimentar y evolution of the lake systems. Consequently, trace fossils can provi de valuable insight for palaeoenvironmental analysis of at least some evaporite formations that accumulated in continental settings. (C) 199 8 Elsevier Science B.V. All rights reserved.