<xs:simpleType name="LithostratigraphicRank">
  <xs:annotation>
    <xs:documentation>Specifies the unit of lithostratigraphy.</xs:documentation>
  </xs:annotation>
  <xs:restriction base="eml:TypeEnum">
    <xs:enumeration value="group">
      <xs:annotation>
        <xs:documentation>A succession of two or more contiguous or associated formations with significant and diagnostic lithologic properties in common. Formations need not be aggregated into groups unless doing so provides a useful means of simplifying stratigraphic classification in certain regions or certain intervals. Thickness of a stratigraphic succession is not a valid reason for defining a unit as a group rather than a formation. The component formations of a group need not be everywhere the same.</xs:documentation>
      </xs:annotation>
    </xs:enumeration>
    <xs:enumeration value="formation">
      <xs:annotation>
        <xs:documentation>The primary formal unit of lithostratigraphic classification. Formations are the only formal lithostratigraphic units into which the stratigraphic column everywhere should be divided completely on the basis of lithology. The contrast in lithology between formations required to justify their establishment varies with the complexity of the geology of a region and the detail needed for geologic mapping and to work out its geologic history. No formation is considered justifiable and useful that cannot be delineated at the scale of geologic mapping practiced in the region. The thickness of formations may range from less than a meter to several thousand meters.</xs:documentation>
      </xs:annotation>
    </xs:enumeration>
    <xs:enumeration value="member">
      <xs:annotation>
        <xs:documentation>The formal lithostratigraphic unit next in rank below a formation. It possesses lithologic properties distinguishing it from adjacent parts of the formation. No fixed standard is required for the extent and thickness of a member. A formation need not be divided into members unless a useful purpose is thus served. Some formations may be completely divided into members; others may have only certain parts designated as members. A member may extend from one formation to another.</xs:documentation>
      </xs:annotation>
    </xs:enumeration>
    <xs:enumeration value="bed">
      <xs:annotation>
        <xs:documentation>The smallest formal unit in the hierarchy of sedimentary lithostratigraphic units, e.g. a single stratum lithologically distinguishable from other layers above and below. Customarily only distinctive beds (key beds, marker beds) particularly useful for stratigraphic purposes are given proper names and considered formal lithostratigraphic units.</xs:documentation>
      </xs:annotation>
    </xs:enumeration>
  </xs:restriction>
</xs:simpleType>