A Silurian-Devonian Transitional Mud-Mound from the Nowshera Limestone, Pakistan

The Silurian-Devonian transition represents a critical interval in the reorganization of early Paleozoic reef ecosystems, marked by a shift from microbial- and mud-dominated buildups to more rigid coral-stromatoporoid framework reefs. Transitional reef types documenting this ecological transfomation, however, remain comparatively rare, particularly along the northern Gondwanan margin. Here we document a mud mound from the Nowshera Limestone of Pakistan, stratigraphically positioned between the Khandar Phylite and the Misri Banda Quartzite, and evaluate its significance for reef evolution during the Silurian-Devonian transition. The studied succession comprises 44 m of carbonate rocks. The lower 24m consists of bedded limestone that transitions upward into a 20m thick massive limestone unit representing the mound body, indicating a shift to a calm, low-energy depositional environment. The mound is dominated by micritic fabrics with abundant stromatolitic and clotted microstructures. Identifiable biota includes Mastopora, Thamnopora, stromatolites, branching bryzoans and sparsely distributed crinoids; fossil preservation is locally compromised by pervasive dolomitization. Microfacies analysis shows that the buildup lacks a rigid skeletal framework and was constructed through microbial binding and baffling, sediment trapping, and stabilization by sessile metazoans, with echinoderms acting as subsidiary anchoring and baffling elements. This buildup represents a mixed microbial-metazoan mud mound occupying an intermediate position along the mud mound-reef continuum. Auloporid (Mastopora) and favositid (Thamnopora) corals primarily functioned as bafflers and sediment stabilizers, whereas stromatolitic microbial communities served as the principal binding agents; crinoids provided additional anchoring and baffling. As part of one of the earliest Paleozoic reef belts recognized on the Indo-Pakistan subcontinent, this example provides important insight into the paleoecological organization and construction mechanisms of transitional reef systems during the Silurian-Devonian transition.