Control of Sediment Supply Variations on Lower Eocene Delta Sequences

Academic Background

In geological research, the formation of depositional sequences is often attributed to changes in accommodation space, while variations in sediment supply are frequently overlooked. However, changes in sediment supply may play a significant role in the formation of depositional sequences, especially in the context of climate change. This study aims to explore the control of sediment supply variations on lower Eocene delta sequences, particularly in the shallow marine depositional environment of the Tremp Basin (Spain).

The research is based on the following context: During the Early Eocene Climatic Optimum (EECO), global warming, elevated atmospheric CO₂ concentrations, and ice-free poles prevailed. This period was also marked by several hyperthermal events, which led to the release of large amounts of isotopically depleted carbon into the ocean-atmosphere system, influencing global climate and sediment supply. By analyzing the sedimentary record of the Tremp Basin, this study investigates how these climatic events affected delta sequence formation through changes in sediment supply.

Source of the Paper

This paper was authored by Romain Vaucher and colleagues from multiple research institutions, including the Department of Earth Sciences at the University of Geneva, the Institute of Earth Sciences at the University of Lausanne, and the Department of Geology at the Autonomous University of Barcelona, among others. The paper was published online on October 9, 2024, in the journal Geology, titled “Sediment supply variation control on lower Eocene delta sequences (Tremp basin, Spain).”

Research Process and Results

1. Study Area and Stratigraphic Analysis

The study area is located in the Tremp Basin in Spain, a foreland basin on the southern edge of the Pyrenees, formed during the collision of the Iberian and Eurasian plates. The research team measured a 644-meter-thick stratigraphic section near the Río Ésera and described the sedimentary facies, fossil content, and sedimentary structures in detail.

The main stratigraphic units studied include the Morillo Limestone and the Castigaleu Formation, which record shallow marine deposition during the early Eocene (approximately 52.2 to 50.6 million years ago). The research team established the chronological framework of these strata using magnetostratigraphy and biostratigraphy.

2. Sedimentary Facies and Geochemical Analysis

The research team conducted organic carbon isotope (δ13Corg) analysis on 293 rock samples to reconstruct the paleoclimate record. Variations in δ13Corg values were used to identify carbon isotope excursions (CIEs), which correspond to global climatic events such as hyperthermal events. Additionally, the team used Rock-Eval 6 pyrolysis to analyze organic matter content, hydrogen index (HI), oxygen index (OI), and maximum pyrolysis temperature (Tmax) to assess the source and maturity of organic matter.

3. Depositional Environment and Sequence Analysis

The results indicate that the depositional environment of the Tremp Basin was primarily delta front and prodelta settings. Delta front deposits are dominated by thick sandstone layers, while prodelta deposits consist of finer-grained mudstones. The research team identified multiple short-term depositional sequences (S-sequences) and smaller-scale elementary sequences (E-sequences), reflecting the progradation and retrogradation of the delta system.

4. Relationship Between Climatic Events and Sediment Supply

The study found that delta progradation phases were closely associated with negative carbon isotope excursions (CIEs), which correspond to hyperthermal events. These events led to warming and increased precipitation, thereby enhancing terrestrial sediment supply. Conversely, positive carbon isotope excursions corresponded to cooler periods with reduced sediment supply, resulting in delta retrogradation.

Conclusions and Significance

This study demonstrates that during the early Eocene, delta sequences in the Tremp Basin were primarily controlled by variations in sediment supply rather than changes in accommodation space. This finding highlights the significant impact of climate change on depositional systems, particularly under high-frequency climatic fluctuations. The results provide new insights into the formation mechanisms of shallow marine depositional systems and offer important references for future interpretations of depositional sequences.

Research Highlights

1. Dominant Role of Sediment Supply: This study systematically demonstrates, for the first time, the dominant role of sediment supply variations in the formation of delta sequences, especially in the context of climate change.
2. High-Resolution Climate Record: Through δ13Corg analysis, the research team reconstructed a high-resolution climate record and correlated it with global climatic events.
3. Multidisciplinary Approach: The study combines sedimentology, geochemistry, and stratigraphy to provide a comprehensive understanding of the mechanisms behind sequence formation.

Additional Valuable Information

The study also reveals the direct impact of hyperthermal events on sediment supply. These events significantly increased terrestrial sediment input by altering hydrological cycles and erosion rates. This finding provides new evidence for understanding the long-term effects of global climate change on depositional systems.

Through detailed sedimentary records and geochemical analysis, this study highlights the critical role of sediment supply variations in the formation of delta sequences, offering important theoretical support for future sedimentological research.