Impacts of Tectonic Subsidence on Basin Depth and Delta Lobe Building. Issue 2 (3rd February 2023) - Dong, Tian Y.
Abstract: Channel avulsions on river deltas are the primary means to distribute sediment and build land at the coastline. Many studies have detailed how avulsions generate delta lobes, whereby multiple lobes amalgamate to form a fan‐shaped deposit. These studies often assume a steady subsidence and uniform basin depth. In nature, however, lobe building is disrupted by variable subsidence, and progradation of lobes into basins with variable depth: conditions that are prevalent for tectonically active areas. Herein, we explore sediment dispersal and deposition patterns across scales using measurements of delta and basin morphology compiled from field surveys and remote sensing, collected over 150 years, from the Selenga Delta (Baikal Rift Zone), Russia. Tectonic subsidence events, associated with earthquakes on normal faults crossing the delta, displace portions of the topset several meters below mean lake level. This allogenic process increases regional river gradient and triggers lobe‐switching avulsions. The timescale for these episodes is shorter than the predicted autogenic lobe avulsion timescale. During quiescent periods between subsidence events, channel‐scale avulsions occur relatively frequently because of in‐channel sediment aggradation, dispersing sediment to regional lows of the delta. Avulsion settings for the Selenga Delta preserve discrete stratal packages that could contain predominately deep channels. Exploring the interplay between tectonic subsidence and sediment accumulation patterns will improve interpretations of stratigraphy from active margins and basin models. Plain Language Summary: River deltas distribute sediment and build land in coastal regions by abruptly shifting course through a process called channel avulsion. The fan‐shaped morphology of river deltas arises from multiple avulsion events that deposits sediment in a radial fashion over time. Our understanding of how deltas build such morphology assumes that size of the downstream basin, such as a lake or ocean, is constant over time. However, geological activities like earthquakes and basin subsidence alter the size and shape of the delta. We complied and analyzed 150 years of delta morphology data from the Selenga Delta (residing in a tectonically active area) in Russia to understand how earthquakes and subsidence impacts channel avulsions and delta landforms. We determined two distinct avulsion styles for the Selenga Delta: a regional‐scale avulsion that is driven by earthquakes, and a local‐scale avulsion that is caused by sediment deposition. The two distinct scales of avulsions blend over time to shape the delta system. In addition, the regional‐scale avulsion produce unique subsurface records that can be used to understand the history of a delta. Our work highlights the importance of understanding the variety in downstream basin processes that impact delta morphology. Key Points: Tectonic subsidence produces variable receiving basin depth, and drives lobe‐scale avulsions by modifying delta‐topset gradient Channel‐scale avulsions occur during periods of tectonic quiescence, and disperse sediment to nourish the deltaic shoreline Hierarchical avulsion styles could lead to preservation of discrete stratal packages that contain predominately deep channels