Millimeter-scale UAV geomorphometry reveals the organization of Neoproterozoic stromatolitic bioherms: evidence from the Irecê Basin, Brazil

Abstract

High-resolution UAV photogrammetry provides an opportunity to examine how primary depositional organization and post-depositional deformation are expressed across scales in ancient microbial carbonate systems. Here, millimeter-scale UAV geomorphometry is integrated with sedimentological and structural observations to investigate Neoproterozoic stromatolitic bioherms at Fazenda Arrecife, Irecê Basin, Brazil. Individual stromatolites are predominantly circular to subcircular in plan view, whereas metre-scale bioherms display greater morphological anisotropy and a significant preferred NE–SW orientation centered near N065°–N245°. Directional concentration increases among the more elongated bioherms, indicating systematic organization at the bioherm scale. Independently measured sedimentary-flow indicators preserved within hummocky cross-stratified deposits define a predominantly N–S orientation and therefore do not show a simple parallel relationship with bioherm elongation. Structural observations provide independent evidence of post-depositional modification: primary stratification is locally folded, deflected, truncated, and displaced adjacent to bioherm margins, together with reverse faulting and distinct fold-hinge orientation families. However, the dominant bioherm axes do not simply reproduce the measured fold or fault orientations, precluding their interpretation as direct tectonic strain markers. The combined evidence therefore supports a superposition model in which primary microbial architecture was subsequently modified and locally amplified by heterogeneous deformation, with gravitational disruption contributing to the geometry preserved at some exposures. These results demonstrate that morphological anisotropy alone cannot uniquely distinguish depositional from tectonic controls in ancient microbial buildups. Integrating millimeter-scale UAV morphometry with independent sedimentological and structural constraints provides a robust framework for separating primary architecture from later modification in deformed carbonate successions.