Depositional dynamics and self-organization in travertine sedimentary systems

Travertines are terrestrial sedimentary systems associated with flowing water oversaturated with respect to calcium carbonate. They form terraced wedge-shaped organogenic bodies, fan-shaped in plan view, with internal achitecture characterized by downslope elongated domal structures (mound), juxtaposed by onlap geometries. Internal features of travertine mounds includes both upward decrease (up to subhorizontal) and downhill increase (up to subvertical) of clinostratification angles suggesting progradational mechanisms. The basic components of travertine deposits are aquatic sessile plants and microbes, developing along water flows. Regardless their role in carbonate precipitation, organisms appear as living templates able to organize primary carbonate encrustations along their growth directions. This results in early-lithified skeletal sedimentary bodies with rapid upward growth. Travertine accumulation transforms original slopes into gently inclined flat areas (travertine terraces), limited downhill by steeper slopes, eventually evolving in subvertical escarpments. Both terraces and escarpments are depositional rather then erosional features, being geomorphic expression of very shallow lacustrine deposits and waterfall structures respectively. Modern to fossil comparison among travertine systems located in southern and central Italy suggest a sedimentary model based on continued feedback between processes and products, which increase the complexity of depositional system over time. Encrusting waters display chemical gradients along their flow, modulating shape and downhill development of resulting travertine deposits. Upward growth gradually decreases original slope angles, so that the water flow is laterally displaced toward adjacent areas of steeper slope, accounting for juxtaposition of travertine mounds. By means of continuous lateral shifting of encrustation process travertine deposition gradually transform original slopes in gently inclined flat areas limited downhill by steeper slopes. This in turn results in a new sedimentary environments, including ponds and shallow lakes in the flattened areas, and waterfalls along the steeper and steeper downhill edge of the travertine prisms. The sedimentary organization of travertine deposits points to spatial and temporal patterns resulting from dynamics internal to the system. This imply self-organization and non-linear bio-chemical deposition rather then external forcing.