Origin and significance of lamination in Lower Cretaceous stromatolites and proposal for a quantitative approach

Abstract

Stromatolite lamination is typically defined as alternation of dark and light laminae. Study of Lower Cretaceous stromatolites fromthe Leza Fm(N Spain) supports this statement, but recognises additional complexities in lamination that have implications for interpreting accretion processes. These stromatolites are partial analogues of present-day coarse-grained carbonate stromatolites in the Bahamas and Shark Bay (Australia) that mainly form by trapping and binding carbonate sand. The Leza examples contain both grain-rich and micrite-rich laminae with scarce particles, suggesting that they accreted both by trapping and not trapping grains. Lamination in modern and ancient coarse-grained stromatolites is commonly defined by thinmicritic crusts that formed during interruptions in accretion and separate contiguous grainy laminae (repetitive lamination). Leza examples also contain these thin hiatal crusts and locally showrepetitive lamination, but their conspicuous macroscopic lamination is defined by thicker alternating grain-rich and micrite-rich laminae (alternating lamination). This indicates that, although hiatuses in accretion occurred, change in accretion process was the main cause of macroscopic lamination. These differences in accretion processes and lamination styles between Leza examples and modern coarse-grained stromatolites may reflect differences in their environmental settings. Modern examples occur in shallow marine tidal environments, whereas Leza Fm coarse-grained stromatolites developed in ideinfluencedwater-bodies in coastal-wetlands that experienced fluctuations inwater salinity and hydrochemistry. Analysis of lamina-thickness in these Cretaceous stromatolites and similar published examples provides a quantitative approach to the processes that underlie stromatolite lamination.