PROPOSED NGDC/CURATORS' CLASSIFICATION
FOR GRANULAR SEDIMENTS
(MODIFIED FROM THE ODP SEDIMENT CLASSIFICATION SCHEME)
Objective: Modify the ODP Sediment Classification System so that it may reasonably be applied to sediment descriptions in the NGDC/Curators' database. (The current ODP classification scheme, developed by Jim Mazzullo, Audrey Meyer and Robert Kidd appears as Appendix B. "New Sediment Classification Scheme" in the ODP Shipboard Scientist's Handbook, March 1990, pp.125-130.)
Approach: Generally accept the naming conventions of the current ODP classification scheme. Although often modified by individual ODP shipboard parties, the underlying premises and structuring of sediment names areis in general use by the majority of by sea-going sedimentologists.
Considerations: To incorporate the ODP scheme into the NGDC/Curators database we would need to
CLASSES AND COMPONENTS
(All capitals in the following text distinguishes operative from explanatory material.)
The ODP classification scheme recognizes two basic sediment types (1) granular and (2) chemical. sediment. The following addresses only the classification of granular sediments, those in which approximate percentages of individual components may determined from gross or smear-slides inspection. Cores which contain more than 50% of precipitates, recrystallized rock or carbonaceous sediments may be referred to the ODP chemical-sediment classification.
Three classes of granular sediments are recognized: Mineral/Lithic, Glass and Biogenic. These equate roughly to Siliciclastic, Volcaniclastic, and Pelagic classes, respectively of Mazzullo, et al., but are redefined to create a wholly descriptive classification, based solely upon grain-size and sediment constituents. Although two classes (Glass and Biogenic) imply provenance (volcanic origin in the case of glass; organic origin in the case of biogenic), the components can be placed in the various classes on the basis of their observed shapes, habits and optic properties without resort to prior knowledge of their provenance.
Biogenic (Pelagic and Neritic, in part, of Mazzullo, et al.)
Def. Remains or traces of once-living organisms.
Discussion: The individual components comprising the class may be recognized on the basis of general form and optic properties, so although "once-living" implies genesis, the describer can recognize and assign these components to the Biogenic Class without a priori knowledge of "once living."
Foraminifer tests and fragments
Diatom frustules, resting spores
Calcareous rock fragments with recognizable biogenic components
Calcareous spines and plates
Plant parts (if comprise less than 50%)
Glass (Volcaniclastic, in part, of Mazzullo, et al.)
Def: Natural and altered glass
Discussion: Mazzullo, et al. define volcaniclastic as derived from volcanic sources and include volcanic detritus "eroded from volcanic rocks by wind or water." I have restricted this class to glass and palagonite, placing the amphiboles and pyroxenes in the Mineral/Lithic Class, as like quartz and feldspar they may be eroded from a variety of igneous sources. The Glass Class is nearly monomineralic, and perhaps over emphasized in this classification. In order to be consistent with the ODP classification, however, and to maintain the commonly used glass descriptors (ash, tuff, lapilli), the class is retained here as a separate category. Although most natural glass is of volcanic origin, mikrotectites, presumed impact-melted spherules have a different origin.
Mafic volcanic glass
Acidic volcanic glass
Mineral/Lithic Class (Siliciclastic, Neritic, in part, and possibly Volcaniclastic, in part, of Mazzullo, et al. The name, Mineral/Lithic, was suggested by Guy Rothwell.)
(Excludes glass and microfossils.)
Def: All mineral grains (clastic, authigenic and precipitates) not included in other granular sediment classes. (Precipitates and carbonaceous materials occurring in quantities greater than 50% would be classified in the "Chemical Sediments" of the ODP Classification.)
Discussion: The term "clastic" is replaced with mineral/lithic as "clastic" implies sediments are detrital eroded and redeposited from pre-existing rocks, a provenance consideration, and does not allow inclusion of authigenic products (e.g. authigenic clay minerals, zeolites, some iron oxides). Distinguishing authigenic from detrital clay is beyond the scope of most preliminary core examinations, and, in any event, all clays need to be grouped into the same category to preserve the customary usage of the "clay" term. "Silici" is dropped because although the bulk the class comprises silicate minerals, some non-silicates are included.
Feldspars (plagioclase, K-feldspars)
Ferromagnesian minerals (amphiboles, pyroxenes and olivines)
Micas (biotite, muscovite,)
Heavy minerals (garnet, tourmaline, zircon, rutile, barite, apatite)
Zeolites (phillipsite, clinoptilolite)
Metallic extraterrestrial grains
Rock fragments (excluding glass, tuff: including coarse-grained recrystallized carbonates, small igneous rock fragments)
Mn nodules, crusts
Neritic Class of Mazzullo, et al. The Neritic Class is dropped from this revised scheme for NGDC data so that all the biogenic components are grouped together and also to eliminate any provenance interpretation. Recognizable biogenic calcareous fragments and coarse-grained calcareous rock fragments are placed in the Biogenic Class and oolites into the Mineral/Lithic Class. This results in some loss of descriptive detail as the recrystallized calcareous rock would herein be termed "limestone" as the primary sediment name. The more precise name involving fabric considerations, packstone, grainstone, etc., could be added to the comment column, as well as to the NGDC list of rock codes.
Mixed Sediments of Mazzullo, et al. The Mixed Sediments Class is also dropped from this scheme. As a principal name, "mixed sediment" does not seem to relay much useful information; the modifiers attached to the principal name should sufficiently indicate various mixtures. "Mixed" when applied to sediments from mixed environments would, in any event, be inappropriate in this descriptive classification.
Class Ternary Diagram - The three classes within the Granular Sediment Type are defined by the ternary diagram below. Note that if the percentage of a single fossil group, (e.g. foraminifers) or fossils of a single mineral composition (e.g. radiolarians, diatoms, and silicoflagellates combined comprise 30% of the sediment, then the biogenic names (ooze, chalk, etc.) are applied. If combining the percentages of all the fossil components, then 50% is required to apply the Biogenic terms. (Because this diagram lumps all of the biogenic components at one apex, a 30% cut-off would be too low to be consistent with standard usage which only applies "ooze" if a single fossil group comprises 30% of the sediment. I have expanded on this slightly to allow grouping of a fossils of the same chemical compositions to be classified an ooze with 30% composition. (e.g. combined foram tests and nannoplankton may be termed a "Foraminifer/Nannoplankton Ooze.")
Following the general conventions of the Marzzullo et al.,/ODP classification, granular sediments are classified by designating a principal name and major and minor modifiers. The principal name is derived by (a) determining the percentages of components to determine granular-sediment class (ternary diagram, above) and (b) selecting the appropriate name from the list unique to that class, (see below). The major and minor modifiers give additional component, and grain-size information. Major modifiers (components) comprise more than 25% of the sediment in the Glass and Mineral/Lithic Class and precede the principal name; minor modifiers (components) comprise 10-25% of the sediment in the Glass and Mineral/Lithic Classes and are designated by "with" following the principal name, (e.g. quartz sand with feldapar.) Because only 30% of a biogenic component is required for the principal name in the Biogenic Class, more than 15% qualifies as a major modifier and 5-15% qualifies as a minor modifier. Some opportunistic descriptors may be added, i.e. "well sorted," "bimodal," "highly fragmented," but for naming simplicity, and potential computer derivation, more detailed observations and descriptions are best placed in the "comments" field.
Biogenic Class - name comprises principal component (e.g. radiolarian, foraminifer) followed by term indication degree of compaction and recrystallization.
Ooze - unconsolidated sediment comprising calcareous or siliceous biogenic-class components.
Chalk - very firm, earthy, fine textured calcareous sediment. Original biogenic components may be broken and compacted.
Limestone - hard recrystallized calcareous rock.
Radiolarite, Diatomite and Siliceous Spiculite - very firm siliceous biogenic-class sediment composed primarily of radiolarians, diatoms, or sponge spicules, respectively.
Chert - Very, dense, hard sedimentary rock composed of siliceous components. (Chert is included here, although it is not necessarily formed of biogenic silica to conform to standard usage and grouping.)
Modifiers for the Comments Field in NGDC Format: Additional grain-size terms are useful in characterizing a sample (e.g. foram "sand.") But because the texture terms "sand, silt and clay" are restricted to Mineral/Lithic Class names, use them in quotes in the Comment Field, not in the "Sediment Name" field. Likewise, other descriptive terms outside this classification scheme may be placed in the Comments Field for greater detail of description.
Glass Class - The principal name is usually based upon grain size. Glass can be recognized by its general shape, structure and optic properties irrespective of knowledge of its genesis. In fact, most glass is of volcanic origin and in the interest of preserving commonly used descriptors "volcanic glass" may be used, if the core describer chooses to make that interpretation. Otherwise, simply describe as glass shards (angular fragments) or spherules (rounded grains).
(Volcanic) ash - angular glass shards less than 2mm in diameter
(Volcanic) lapilli - glass shards between 2 and 64mm in diameter
(Volcanic) tuff - friable to firm welded ash
Breccia - cemented angular glass fragments greater than 64mm in diameter
Spherules - rounded glassy grains (microtektites)
Mineral/Lithic Class - The sediment name comprises principal component (e.g. quartz) and grain size descriptor (e.g. sand, silty clay, etc. as determined form Udden-Wentworth's grain-size scale (1922) and Shepard's (1954) ternary diagram. (This presumes replacing the Folk diagram in the current NGDC format with Gravel, Mud or Ooze and Sand at the apices (Data Announcement 86-MGG-10) with Shepard's "Sand, Silt and Clay" diagram. Attach "stone" to the recrystallized hard rock of the same composition and grain size. For firm, crumbly, but not recrystallized sediments, modify with "friable," (e.g. friable sandstone).
Terms for texture applied to sediments of the Mineral/Lithic Class. Determine percentage of grains in each size category and select name from the Ternary Diagram after Shepard shown below.
Unconsolidated Hard Rock
Sandy clay Sandy claystone
Silty clay Silty claystone
Clayey sand Clayey sandstone
Clayey silt Clayey siltstone
Silty sand Silty sandstone
Sandy silt Sandy siltstone
Other widely used terms such as "manganese nodules," "glauconite pellets" are acceptable, but the describer is encouraged to indicate their approximate size as these terms are not defined in the Wentworth scale. Use of mudstone is discouraged as the relative proportion of silt and clay is not difined.
Modifiers for the "Comment Field": Further descriptive detail involving grain fabric, degree of recrystallization, sphericity and roundness of grains, can be added in the Comment Field. Sediment color (in the NGDC format) is entered in a separate field, though some users find that giving the plain English equivalent of color(s) in the comment field is helpful.
Major Grain-Size Classes after Wentworth (1922)
Wentworth Size Class Millimeters
Sand .06- 2 (.06 rounded from .0625)
Silt .039 (rounded to .004)
Clay Less than .004
(See Wentworth, 1922 or Mazzullo, et al. for size ranges of the "fine, medium and coarse" subdivisions of principal grain-size classes.)
Shepard's (1954) Ternary Diagram
Apply to determine sediment names in the Mineral/Lithic Class
This granular classification scheme is oriented to the description and classification of soft sediments, though is also applied to broad categories of sedimentary rock (siltstone, limestone, chert). A more refined classification of sedimentary rocks, recognizing degree and type of compaction and recrystallization and involving fabric descriptions is desirable, but beyond the scope of this particular revision. For the time, the NGDC list of rock names can be expanded to accommodate additional rock names.
4 August 1998