Origin of spheroidal weathering within terra rossa over limestone

Abstract

While the process of spheroidal weathering in silicate rocks has been studied intensively, much less is known about this process in carbonate rocks. Here we report an unusual spheroidal weathering phenomenon within terra rossa overlying the dolomitic limestone bedrock of the Yunnan-Guizhou Plateau in southwestern China. The spheroidal weathering products are characterized by iron shells. Restoration of the protolith reveals that the parent rock of the spheroidal weathering products is silty carbonate rock, rather than pure carbonate rock. Our results also show that the formation conditions of the spheroidal weathering include the following: (1) the outer iron shell supports the weathered rock body and prevents collapsing; (2) the silty carbonate rock contains sufficient silicate components to form the matrix and framework of the spheroidal weathering products; and (3) the inward diffusion of external Fe(II) promotes rindlet formation. The inward diffusion of Fe(II) and the associated ferrolysis weathering are responsible for the quasi-synchronous Fe-(hydr)oxide precipitation and dissolution of carbonate minerals. Regarding hydrogeological conditions, spheroidal weathering may occur in a paleo-perched saturated zone or in zone of fluctuating paleo-phreatic water in the epikarst zone. This redox transition zone facilitates sustainable ferrolysis weathering and rindlet formation. In general, the development of spheroidal weathering patterns comprises four stages: (1) development of incipient joints and fractures, (2) formation of an initial outer iron shell, (3) early rindlet development, and (4) a late karstification process (with inactive spheroidal weathering).