STORM-DOMINATED DELTAS PRIOR TO THE EVOLUTION OF LAND PLANTS: HIGH-VOLUME, IN-SITU CLAY MINERAL FACTORIES?

Secular changes in clay mineral abundance and mineralogy are believed to represent excellent proxies for long-term climate change, sediment provenance and organic carbon burial efficiency. Prior to the evolution of widespread terrestrial soils, high volumes of reactive minerals did not completely weather on land but were sourced, often non-weathered, to the world’s oceans. In result, a potentially large volume of the neo-formed clay fraction precipitated within buried sediment – entirely disconnected from weathering processes in the hinterland. In this study we quantify the ratio between diagenetic (in-situ) versus detrital clay minerals within an early Paleozoic, storm-dominated delta that was deposited prior to the evolution of land plants. Systematic sedimentological and compositional analyses of mud and sandstone in the early Ordovician (~385 Ma) Power Steps Formation, Bell Island (Newfoundland) reveal that in the present-day clay mineral fraction illite is often detrital whereas chlorite originates preferentially from the breakdown of highly unstable, volcanoclastic lithoclasts. Detailed petrographic examination reveals that these lithoclasts were diagenetically altered to chlorite before significant compaction, thereby yielding significant volumes of new, pore-bridging chlorite and silica cement that entirely eliminate intergranular porosity. Sandstones are preferred sites of late-diagenetic chlorite precipitation, while diagenetic illite preferentially precipitates in mudstones. A large fraction of compositional and diagenetic rock attributes varies as a function of starting composition (provenance), hydrodynamic sorting and grain stability. Future studies must consider the yet underappreciated impact of “in-situ weathering” when employing whole-rock X-ray diffraction as a proxy for provenance, climate change and hinterland weathering dynamics.