Stratigraphy

This document contains guidance for managing stratigraphy data in EMu. This workflow was originally developed for a data cleanup project in 2021.

Some goals of this workflow include:

Standardizing names of stratigraphic units to improve recall
Identifying and moving data that is in the wrong field
Linking units to Macrostrat records or other authorities

Stratigraphy data standard

Stratigraphy data is stored in the Stratigraphy grid in the Catalog > Stratigraphy tab. This standard defines how stratigraphic data should be entered in those fields.

Authorities

The best source for verifying stratigraphic names is probably Macrostrat, which is a curated database of units that includes information from both the literature and major geological surveys (primarily the USGS). Macrostrat does a nice job depicting relationships between units and alternative names that describe the same unit. It also makes data about stratigraphic units available through an API.

Other good resources are websites for country geological surveys, like the U.S. Geologic Names Lexicon. For many units, Macrostrat has already compiled data from these surveys, but sometimes the original records have useful information that was omitted.

Ranks

Stratigraphic units are arranged in a hierarchy. From least to most specific, the typical ranks and abbreviations are:

Supergroup (SGp)
Group (Gp)
Subgroup (SubGp)
Formation (Fm)
Member (Mbr)
Bed (Bd)

There may be others. People like their subdivisions.

You will also see the term “series” in some places. This is an informal rank but generally refers to either a formation or group. You can, for example, safely standardize “Honolulu Volcanic Series” to “Honolulu Volcanic Fm” since the latter is the form used by Macrostrat.

EMu includes fields for Group, Formation, and Member. Unit names should always go in the field matching their rank and should include the abbreviated form of the rank:

Glen Canyon Group ⇒ Group = Glen Canyon Gp
Navajo Formation ⇒ Formation = Navajo Fm

Populating higher ranks: Macrostrat and other sources may include higher stratigraphic ranks that are missing from EMu. If you are able to confidently identify a missing parent group or formation, you can add the parent to EMu in the appropriate field. For example, Macrostrat notes that the Navajo Formation is part of the Glen Canyon Group. That group name can be added to any records that do not already include it. If there’s any ambiguity to the match, add a note.

Inconsistent higher ranks: If an authority specifies a different parent than is recorded in EMu, note that the hierarchy is inconcistent with that authority.

Multiple ranks in the authority record: Sometimes the authority record will include alternate names of different rank (for example, https://macrostrat.org/sift/#/strat_name_concept/6602). For Macrostrat records, defer to the name/rank with the higher number of citations. For other sources, note the multiple ranks.

Supergroups, subgroups, and beds

Some stratigraphic ranks do not have dedicated fields in EMu, Darwin Core, or both. The general guidance for these ranks is to include them only if they appear in the original EMu record. Here are the possible cases:

Supergroup but no group: Group = Supergroup SGp
Supergroup and group: Group = Supergroup SGp > Group Gp
Group and subgroup: Group = Group Gp > Subgroup SubGp
Subgroup but not group: Group = Subgroup SubGp
Member and bed: Member = Member Mbr > Bed Bd
Bed but no formation: Member = Bed Bd

Only populate missing values for these ranks if there is no data in the relevant field.

Lithologies

Unit names sometimes include the name of the primary rock name that makes up the unit. Some rock names will be familiar (limestone), some less so (tonalite). Sometimes rock names are abbreviated. Common abbreviations include:

Ls: Limestone
Ss: Sandstone or siltstone

Rock names are used inconsistently in stratigraphic units, and the same unit may appear in the literature with or without one. For units that grade between lithologies (for example, from sandstone to siltstone), the rock name may add a bit of extra info that you’d lose if you omitted it, so the best practice is to defer to the data in EMu or the original documentation. If those records includes the rock name, keep it when standardizing the name:

Navajo Sandstone ⇒ Navajo Sandstone Fm

Otherwise leave it out:

Navajo Formation ⇒ Navajo Fm

You can include the rock name even if it isn’t officially listed by Macrostrat as an alternative name. Note also that rank (“Fm” in this case) is included in either case.

The diversity of rock names is large and the abbreviations are less than perfectly clear, so for clarity you should always use the full rock name if you do decide to include it:

Navajo Ss ⇒ Navajo Sandstone Fm

Modifiers

Unit names in EMu may be modified to describe where in the unit a given sample was collected. Common modifiers include:

Top
Upper
Middle
Lower
Bottom

To maintain sorting in the lookup list, include modifiers as a trailing parenthetical:

Upper Navajo Formation ⇒ Navajo Fm (Upper)

Ranges

Some records include stratigraphic ranges. For ranges, each unit should get its own line in the Stratigraphy grid, with the older unit listed first.

Cleanup workflow

Use the following steps to clean stratigraphic data:

Verify the stratigraphic data
Match stratigraphic name to an authority
Update the EMu record

1. Verify the stratigraphic data

The Stratigraphy and Geologic Setting fields contain similar data, and as a result, a name will sometimes end up in the wrong field.

Stratigraphy should only be used for stratigraphic units (groups, formations, members, etc.), that is, names that refer to horizons in the rock record.
Geologic setting should only be used for geologic place names (batholiths, provinces, etc.) These are geographic names referring to geologic features.

So the basic difference is that stratigraphy refers to the rock record, whereas setting refers to a point or area on a map.

Address problem data as follows:

If data in the Stratigraphy grid is not a stratigraphic unit, figure out what kind of feature it refers to and move the data to the proper field. This will probably involve updating the Collections Event record. Be sure to search for the unit name in Macrostrat before making this determination.
If data in Geologic Setting is a stratigraphic unit and the Stratigraphy grid is empty, move the data in setting to the appropriate field in Catalog.
Stratigraphic data is often included in both the Stratigraphy grid and Geologic Setting. If both fields contain the same information, determine which one should be use and clear the other.
If the Stratigraphy grid and Geologic Setting contain different stratigraphy information, use primary collection documentation to sort out which value is correct. Make sure to note the incorrect data in a note.

Terms that are likely to be units

Amphibolite
Andesite
Argillite
Basalt
Claystone
Conglomerate
Dacite
Eclogite
Gabbro
Gneiss
Granite
Granulite
Greenstone
Greywacke
Limestone
Marl
Marble
Meta-
Mudstone
Porphyry
Sandstone
Siltstone
Tonalite
Trondjemite
Tuff
Welded Tuff
Most other rock types, there are lots

Terms that are likely to be settings

Alnoite
Batholith
Diatreme
Kimberlite
Pipe
Province
Volcano

Note: Kimberlite, lamproite, alnoite, and a few others are rock types, but names using these terms usually refer to a pipe (basically a special type of volcano) where material was erupted. Macrostrat sometimes include such pipes as units, but I think they are closer to settings.

2. Match stratigraphic name to a Macrostrat record

Stratigraphic names are not unique. When researching a unit, make sure to verify that all metadata (notably geography and higher stratigraphy) for that unit matches the record in EMu.

If you can match the stratigraphy info to an authority, record the link in a note. Macrostrat is the preferred authority, but if no record exists there, you can link to another resource as long as the link resolves to a specific record for the current unit.

3. Update the EMu record

Ensure names are in correct fields
Standardize names to preferred format (defined below)
Fill in higher stratigraphic ranks, if possilbe

Examples

Real stratigraphic data can be tricky to parse and clean. The examples below illustrate some some common issues you’re likely to run into when working with this kind of data.

NMNH PET 102574 

Field	Original	Recommended
Group	Dakota Ss.
Formation		Dakota Sandstone Fm
Member

The unit is originally given as Dakota Ss. To get the final value, we expand the rock type and append formation to the original value, yielding Dakota Sandstone Fm. The tricky part with this one is that the lithology and rank of this unit vary over time, space, and by author, and there is no clear best option for a canonical name or rank. That’s fine, really–one of the reasons we use an authority in the first place is to help sort out complex historical usages–but since EMu uses a hierarchy for stratigraphy we do ultimately have to pick a rank for this unit, ideally one that can be used consistently across the database (or, even better, across the discipline).

Macrostrat lists four names under Dakota:

Dakota Fm
Dakota Gp
Dakota Sandstone Fm
Dakota Conglomerate Fm

The USGS Geolex entry gives the same four names and includes a list of formations and members that are associated with the Dakota unit in the literature.

When selecting a name and rank, consider:

Does the database include children of these units? What are the ranks of those children? If the children are all formations, it may make sense to treat the parent unit as a group. If they’re all members, formation may make more sense.
Do database entries for the name commonly specify a rock type? Groups are typically diverse and don’t include rock types, so that may mean that formation is a better home for this data.

There’s not really a wrong answer, and it’s worth noting that Geolex and Macrostrat often do not specify rank on the titles of their unit pages, in part because the rank question cannot always be resolved. The main thing to aim for is consistency.

NMNH PET 117710-42 

Field	Original	Recommended
Group	Beaver Bay Group	Beaver Bay Gp
Formation	Stepovak Formation - Boulder Bay-Fox Bay Section	Stepovak Fm > Boulder Bay-Fox Bay Section
Member

A section is a specific exposure of a unit. Stratigraphic units can be horizontally extensive and are not necessarily homogenous, so it can be useful to note where in the unit a rock was sampled from. Section names are often project-specific and are unlikely to appear in central authorities. If you find a section name, append it to the most specific unit name, e.g. “Stepovak Fm > Boulder Bay-Fox Bay Section”

NMNH PET 117247-270 

Field	Original	Recommended
Group	Colorado Group	Colorado Gp
Formation	Basal Fall River Formation	Fall River Fm (Basal)
Member

This is an example of multiple, apparently related units being assigned different ranks over time. Colorado Group is by far the most common variant for the Colorado Group/Shale/Formation in Macrostrat, while the Fall River unit is variously listed as a formation or member. The closest match to the EMu records is actually Colorado Fm > Fall River Mbr, but given the clear preference for Colorado Group, I’d map this as Colorado Gp > Fall River Fm (Basal) instead (with basal being a modified of the unit name).

NMNH PET 117230-130 

Field	Original	Recommended
Group	Keres	Keres Gp
Formation	Peralta Tuff	Bearhead Fm
Member		Peralta Tuff Mbr

This is an example of a unit being assigned the wrong rank in EMu. According to Macrostrat, the Peralta Tuff is part of the Bearhead Formation, which is itself part of the Keres Group. So the hierarchy in EMu is consistent with Macrostrat–the Peralta Tuff is a subunit of the Keres Group–but the Peralta Tuff is in the wrong place. The main thing to look for in cases like this is consistency—are the parent-child relationships generally correct, even if there are gaps?

NMNH PET 117227-210 

Field	Original	Recommended
Group	Polvadera	Santa Fe Gp
Formation	Puye	Puye Fm
Member

Samples assigned to Polvadera > Puye are mostly volcanic. The Polvadera Group is a volcanic unit that occurs in the same general area as the Puye Formation, but the two units do not appear to be related. The USGS instead describes the Puye as a conglomerate or gravel consisting of a mix of sedimentary and igneous rocks that is part of the Sante Fe Group. At first glance, that description does not seem like a good match for the rocks in the collection. However, some 1970s publications associated with the Puye (viewable using the Significant Publications link on the Puye page on Geolex) reference one or more pumice layers within that formation. These seem like the most likely source for the NMNH rocks, and that is the interpretation we went with here.

NMNH PET 117795-7 

Field	Original	Recommended
Group
Formation	Kalamazoo Volcanics	Kalamazoo Volcanics Fm
Member	1	1

Numeric members are likely to be project-specific and are unlikely to appear in authorities but might show up in associated publications. As here, we leave them as is. Some numbered units, like zones, might represent biostratigraphic layers.

NMNH PET 117806-946 

Field	Original	Recommended
Group
Formation	Paxton Schist	Paxton Schist Fm (Lower)
Member	Lower

This example demonstrates two decisions we made when mapping units to authorities:

What do we do with unit names that include rock types in EMu but not in the authority? EMu gives the formation here as Paxton Schist, but Macrostrat uses Paxton Fm. We’ve chosen to keep the rock name and append formation. Here is the reasoning behind this decision:
1. For some units, the variant including the rock type is by far the preferred way to refer to the unit. Removing the rock type hampers discoverability. (ex. Bishop Tuff)
2. Units, especially groups and formations, may include multiple rock types, including igneous layers. Keeping the rock type may provide additional context when the sample is different from the parent rock (as in NMNH PET 117227-210 above).
3. Including a link to an authority should resolve any ambiguity from using a slightly different name, so the exact name is not important if the link is provided and active.
What do we do with modifiers that appear in separate fields? The original entry for this sample puts lower in a separate field. We reserve fields for named (or rarely numbered) units, so we appended lower as a parenthetical modifier to the formation name instead. Common modifiers include basal, lower, middle, upper, and top.