Textural transition in an aragonite travertine formed under various flow conditions at Pancuran Pitu, Central Java, Indonesia

Abstract

A large travertine mound at Pancuran Pitu, Central Java, Indonesia, displays various types of lamination reflecting different hydrological conditions. To understand the geomicrobiological processes of the laminated travertines, we investigated the mound's hydrology, water chemistry, texture, microbial composition, and volume percentage of carbonate mineral (VPC). We identified four types of travertines: dense travertine (80% VPC, flow rate ~200cm/s), lithified microbial travertine (60% VPC, flow rate ~70cm/s), fragile microbial travertine (30% VPC, flow rate ~10cm/s), and poorly lithified microbial mat (<10% VPC, flow rate <5cm/s). The first two types exhibited regular lamination at sub-mm intervals that was associated with the distribution of cyanobacteria and organic substances. The dense travertine consisted of alternation of crystalline and banded layers at ~160-μm intervals, which resembles the daily lamination of an aragonite travertine from southwest Japan. The banded layer formed during the day, exhibited porosity, and contained organic substances. Lamination in the lithified microbial travertine was likely formed by the daily growth cycles of filamentous cyanobacteria. The daytime surface of this travertine appeared to be sub-mm-thick cyanobacterial biofilm with spheroidal aggregates of aragonite, which covered a crystalline layer consisting of radially expanded needle crystals. The crystalline layer was likely formed during the night when the cyanobacterial growth was interrupted. These regularly laminated travertines fabrics appeared to be similar to certain Precambrian stromatolites. Regularity of the lamination was less in the other two microbe-rich types at low-flow sites. The majority of the aragonite occurred as spherical aggregates within the microbial mat, although thin (10–50μm) crystal layers in the fragile microbial travertine may indicate daily intervals. As represented in the relationship among the VPC, flow rate, and precipitation rate of aragonite, the travertines were more consolidated with increasing hydrodynamic energy because of the activation of carbonate precipitation and the inhibition of thick cyanobacterial biofilm development.