Distinct grain-size patterns in sandy turbidites: Implications for identifying the triggers of ancient turbidity currents in lacustrine settings

Abstract

Understanding the triggers of turbidity currents from their deposits is crucial to constraining past climate and environmental changes, reconstructing tectonic or seismic activities, and predicting submarine geohazards. However, it is challenging to determine the origin of turbidites when the examined interval cannot be correlated with historically known events or exhibits no lithofacies characteristics. Such work has therefore been performed mainly toward modern turbidites as they are better correlated with recorded events than ancient turbidites. Here we identified two distinct grain-size patterns based on detailed grain-size analysis of the Triassic sandy turbidites in the Ordos Basin. The two patterns vary significantly in sorting and slope value of one-dimensional linear regression equations in the coarsest versus the median grain size (D₉₉–D₅₀) diagram. Poorer sorting with lower slope value (0–1) is interpreted to be river flood-triggered turbidite, whereas better sorting with higher slope value (>1) is interpreted to be slope failure-triggered turbidite. This interpretation is then evaluated by comparison to modern turbidites of known origin. This study suggests that even if the lithofacies are very similar, turbidites with different origin can display distinct grain-size patterns, reflecting fluctuations in source materials, flow capacity and velocity of their original turbidity currents. It also provides a new insight into identifying the triggers of ancient turbidity currents using grain-size data.