The fate of terrigenous organic matter in syn-rift lacustrine and fan-delta systems: from deposition to catagenesis

Wang, Qianyou ORCID: 0000-0001-5158-821X (2025) The fate of terrigenous organic matter in syn-rift lacustrine and fan-delta systems: from deposition to catagenesis PhD thesis, University of Liverpool.

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Abstract

Organic carbon (OC) burial in lakes has been identified as an efficient sink in the global carbon cycle. Abundant input of dispersed terrigenous organic matter (OM) leads to high variability in OM origin and type, but its enrichment in lake basins has been neglected compared to aquatic OM. Here, the Lower Cretaceous Shahezi Formation from the syn-rift section of the Lishu Rift Depression, Songliao Basin, were investigated to reveal the composition, deposition, and thermal transformation of terrigenous OM within a tectono-stratigraphical and palaeoclimate framework. Firstly, a sequence framework and depositional pattern for asymmetric half grabens in syn-rift lake basins are proposed by characterising the lacustrine and fan deltaic deposits. The Lishu palaeo lake is featured by its relatively small size, steep slopes, poorly developed and siliciclastic dominant shoreline strata, and significant input of allochthonous biogenic clasts. A complete cycle of the syn-rift deposits comprises a prolonged retrogradational phase (LST and TST) and a brief progradational phase (HST). The substantial increase in accommodation space, resulting from fault-generated subsidence, triggered lake expansion and further contributed to the mudstone deposition. The offshore mudstones, rich in terrigenous OM and allochthonous fossils, correlate with carbonaceous mudstones, coals, and conglomeratic sandstones in proximal sections, indicating a dynamic interplay between fan delta progradation and lake shoreline transgressions. Secondly, the 90-metre interval dominated by lacustrine mudstones is analysed to reveal the terrigenous OM deposition in relation to syn-rift lake evolution and palaeoclimate change. Multiple independent proxies (organic maceral composition, pyrolysis indices, vitrinite reflectance Ro, TOC/TN, and δ13Corg) suggest that the highly mature mudstones were dominated by terrigenous OM (gas-prone type III/IV kerogen). A direct correlation between depositional facies, chemical weathering proxies, and pyrolysis indices has been established. The high input of terrigenous OM is jointly promoted by a steep syn-rift slope, a warm and humid climate, abundant vegetation, lake shoreline transgressions, and subaqueous transport of OM-laden sediments. At last, a mass balance model with a refined algebraic scheme is proposed to calculate the TOC loss during catagenesis and reconstruct buried OC content. A one-at-a-time sensitivity analysis method was introduced to quantify the propagation uncertainties in the model by examining the responses of TOC restoration outputs (TR, f, and σTOC) to variations in key inputs (TOCpd, BIpd, HIpd, HIo, Cc, α, and β). Both the simulated and experimental results demonstrate that the proposed model improves the TOC restoration accuracy by accounting for the rock mass changes and OC deductions due to hydrocarbon expulsion. The uncertainties arising from S1 “carry-over” and mineral matrix effects are resolved through the new equations. The long-term accumulation and transformation of terrigenous OM in syn-rift lake basins is investigated in this study. The mass balance model and the associated equations can help quantify the original influx of terrigenous OM. The findings offer valuable insights that can complement existing OC enrichment models developed for aquatic OM. As the results also speak to the effectiveness of OC burial in sediments, this study may contribute to a better understanding of the global carbon cycle in geological history.

Item Type:	Thesis (PhD)
Divisions:	Faculty of Science & Engineering Faculty of Science & Engineering > School of Environmental Sciences
Depositing User:	Symplectic Admin
Date Deposited:	14 Aug 2025 09:52
Last Modified:	14 Aug 2025 12:13
DOI:	10.17638/03193317
Supervisors:	Worden, Richard H Mariani, Elisabetta
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3193317
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