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Lithofacies Features and Organic Geochemistry of Salt Marsh-Shallow-Marine Deposits in the Middle Mandano Formation, a Middle Pleistocene on the Boso Peninsula, Japan


This study investigated lithofacies features and organic geochemistry of muddy deposits formed in salt marsh and shallow-marine environments, which are encased between coastal and shelf sandridge deposits of the Middle Pleistocene (ca. 0.6 Ma) Mandano Formation on the Boso Peninsula, Japan. The coastal deposits represent the lower part of the Mandano Formation (up to 40 m thick), and is interpreted to have formed in response to forced regression of a shelf-margin delta. In contrast, the sandridge deposits represent the upper Mandano Formation (up to 65 m thick), and is considered to have formed during a transgressive stage. The encased muddy deposits (up to 17 m thick) represent the middle Mandano Formation, and is defined by an erosional unconformity and a ravinement surface in the base and top, respectively.

The muddy deposits are characterized by siltstones and silty sandstones, and contain many plant fragments in the basal and upper parts. Overall, the muddy deposits are intensely bioturbated upsection and contain shallow-marine molluscan shells in the middle part, indicating an overall transgressive-to-regressive succession from salt marsh to shallow-marine embayment, which formed in response to retrogradation and progradation of a bay-head delta in the southwestern landward area as a response to inundation of sea water into a salt marsh environment. Locally, the muddy deposits are intercalated with tidal rhythmites and hyperpycnites, and also spit deposits in the northeastern distal area, which formed mainly during late transgressive and early regressive stages. In addition, the muddy deposits formed in the southwestern proximal area are represented by gamma-ray intensity higher than those formed in the northeastern distal area, indicating active settling of clay particles in and around river mouth areas during flood stages.

On the basis of the combination among the C/N ratios, δ13C and total organic carbon contents (TOC), salt marsh deposits in the basal part of the muddy deposits and hyperpycnites mainly are represented by higher TOC values (1.5-16%), and by organic carbon derived mainly from terrestrial plants. In contrast, muddy deposits formed during late transgressive and regressive stages are represented much more by organic carbon derived from marine organisms, and their TOC values are less than 1%. Consequently, the salt marsh deposit and hyperpycnites have a higher potential as source rocks in coastal and shallow-marine successions.