2006 VCD JOHN

Core Description during ODP and the early part of IODP: 

Core Description during ODP and the early part of IODP Jay Miller Cédric M. John (USIO) VCD Meeting, College Station, TX, 27th September 2006

GOALS OF CORE DESCRIPTION: 

Characterize a rock material by its composition, texture, color, structure present, remarkable features, etc… Based on the above, assign the rock material to an interpretative category in a specific classification. This step is equivalent to a simplification of the rock system. Rock classification ultimately helps in our understanding of the geologic history of the material/region GOALS OF CORE DESCRIPTION

GOALS OF CORE DESCRIPTION: 

In that sense, the nature of the rock material should be irrelevant to the description process. BUT… Historically (DSDP, ODP, early IODP) hard rocks and sediments have been described following very different procedures, in part dictated by core flow, amount of material recovered, and scientific objectives. GOALS OF CORE DESCRIPTION

Part I: describing sediments: 

We will look at the descriptive core flow and classification schemes of two different expeditions: IODP Expedition 308, Gulf of Mexico: siliciclastic system. ODP Leg 194, Marion Plateau, NE Australia: Mixed siliciclastic-carbonate system. Part I: describing sediments

EXP 308: Gulf of Mexico: 

Expedition 308 recovered sediment from the Mississippi and Brazos-Trinity rivers fan deposits. EXP 308: Gulf of Mexico

Steps taken by sedimentologists: 

Choose a classification scheme based on previously recovered sediments from the GOM. Describe the core material, focusing in particular on the information needed to classify the rock. Record detailed observations on paper ( 1 “barrel sheet” per section) Transfer the information from the barrel sheet to a software (AppleCore). Plot each core (up to 7 sections) information graphically on 1 letter-sized page using AppleCore (summary sheet). Use the summary sheets to group intervals into rock unit and subunits. Steps taken by sedimentologists

EXP 308: Gulf of Mexico: 

modified Shepard (1954) classification EXP 308: Gulf of Mexico

Slide8: 

Barrel Sheet (hand drawn)

Slide9: 

Barrel Sheet (hand drawn)

Slide10: 

Barrel Sheet (hand drawn)

Slide11: 

Descriptive data entered numerically: Smear Slides

LEG 194: Marion Plateau: 

Leg 194 recovered mixed carbonate-siliciclastic sediments as well as carbonate platform sediments LEG 194: Marion Plateau -> two different classification schemes used!

LEG 194: Marion Plateau: 

For mixed-siliciclastic sediment with >25% non-carbonate grains, we used the Shepard (1954) classification -> based on grain size LEG 194: Marion Plateau

LEG 194: Marion Plateau: 

For carbonate sediment with <25% non-carbonate grains we used the Dunham (1962) classification as modified by Embry and Klovan (1971) -> based on texture (relationship of matrix to grain) LEG 194: Marion Plateau

Summary of main challenges facing sediment core description: 

Detailed information is not captured in the database, and is not searchable. The most detailed information is still captured by hand, and until recently was not digitized. Because data are not entered in the database as searchable fields, a lot of information is “lost” to the wider scientific community who may not know where to look for it. Interpretative data (e.g. paleoenvironment, Lithological Unit, …) is not captured in the database. The classification scheme has to be decided prior to recovery, and once agreed upon, may influence the type of information being routinely recorded (e.g. composition vs texture). If more than 1 classification scheme is used, a decision has to be made regarding which one to use when describing an interval. Ideally, description should be as objective as possible and not be based on a preliminary interpretation (e.g. is the sediment carbonate- or siliciclastic-dominated?) Summary of main challenges facing sediment core description