Late Pleistocene travertines and their analogues underdeposition. A comparative analysis

Abstract

Anzalone E.: Late Pleistocene Travertines and Their Analogues Under- Deposition. A Comparative Analysis. (IT ISSN
0394-3356, 2008).
Textures, sedimentary structures and O-isotope data from the late Pleistocene travertines cropping out at Pontecagnano-Faiano
(Salerno) are here compared with carbonate incrustations under formation within an adjacent irrigation channel.
Fossil system. A complex paleo-environmental dynamics is documented in the travertine fossil system, spanning in age from 90 ka
B.P. (±10ka) at the base, to 55ka B.P. (±10ka) at about 15 m from the summit surface. In this system, aggradational and progradational growth-geometries developed starting from an original gentle slope, that was increasing gradually its inclination while growing
downhill. This resulted into a number of rapids and waterfalls at the margins of this system leaving behind flat terrace where swamps
and/or shallow lakes developed. The travertine growth was interrupted, at time, by non deposition and/or erosional processes, both
documented by discontinuity surfaces. This organization suggests a recurrent dropping of the groundwater level and a decreasing
spring-water outflow, considered as due to climatic changes. Moreover, oxygen isotope data have been measured lamina-by-lamina
on stromatolitic travertines in which recurrence of the Chironomid larvae galleries in the sparitic laminae suggests seasonal variations
as also confirmed by the small isotope oscillations (dispersion of about 0,6 ‰ δ units) documented in the laminae of the stromatolitic
fabric.
Present-day incrustations. The present-day incrustations derive from waters flowing along an artificial channel fed by springs that has
been used as a gentle “slope” depositional analogue of the adjacent fossil deposits. The time-space evolution of the depositing incrustations shows growth geometries well comparable to those occurring at a larger scale in the fossil system. The incrustations develop downhill and from the margins towards the channel axis both through progradational and aggradational geometries. Small barrages and domes (mounds) take form where the carbonate calcium precipitation rate is higher, while micropools tend to spread behind these structures. These depositional structures tend to level themselves to the channel upper surface. The oxygen-isotope data measured in the channel incrustations document small seasonal oscillations (dispersion of about 0,6 ‰ δ units) that do not differ from
those recorded in the fossil stromatolitic deposits. Based on the above results a sedimentary model is proposed here, that may bear
also on the terraced morphology of other Quaternary ambient-water travertines