Guidelines for the compilation of the legend

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Legenda

The legend is the most important frame element of a geological sheet because it provides the key to reading the sheet itself. A clear, concise legend exhaustively listing all the basic information required, will facilitate the interpretation and comprehension of the map field as well as of the geological profiles and other marginal diagram.
It is therefore extremely important to bear in mind the type of user – for a geological sheet the pool of users includes all Earth Science technicians and specialists. The compiler must therefore make every effort to draw up the legend using – at least with regard to the basic features distinguishing the mapped stratigraphic units macroscopically – a language that is accessible to everyone and commonly used terms rather than specialist jargon.
Regardless of the way the legend is structured (paleogeographic domains, successions, tectonic units, etc.), the description of the mapped stratigraphic units should always approach the lithology "by degrees", necessarily beginning with macroscopic aspects (lithological types, their relations, colour, thickness and geometry of the strata), eventually proceeding, according to the case in question, to textural types, mineralogical and petrographic associations, primary and secondary structures, mineralisations present, etc. Compilers may include descriptions of stratigraphic boundaries and relations with other stratigraphic units, as well as of the depositional environment and the way their formation took place.
When dealing with sedimentary rocks, attention should be paid to the paleontological association, provided it contributes to the macroscopic characterisation of the unit, while biozones may either be identified in the legend itself or in the marginal stratigraphic diagram.
The thickness of the unit, whether measured or estimated, is of fundamental importance and should be indicated in the legend itself, or if considered appropriate, in the stratigraphic table (for example, F° 197 "Bobbio").
Finally, compilers must take care to mention age, even if it is uncertain. Descriptions of stratigraphic units should only use English terms when there is no Italian equivalent.
The stratigraphic units in the legend should be listed beginning with the most recent and ending with the oldest, even in the case of several successions, domains or tectonic units. In the case of legends structured by tectonic units, the same ones should be listed beginning with the geometrically highest one.
Given that for the purposes of the databank it is essential that each stratigraphic unit is identified by an abbreviation defining it univocally, compilers should follow these guidelines (for general rules see Quaderno, serie III, n. 6):

  1. the supergroup and group are defined by two letters;
  2. the subgroup is defined by a subscript number next to the group abbreviation, numbered consecutively beginning with the oldest;
  3. the formation is defined by three letters;
  4. the member is defined by a subscript number next to the abbreviation of the formational unit, numbered consecutively beginning with the oldest;
  5. formational units of a lower rank (lithofacies, stratum, lense, etc.) are defined by a lower-case letter in subscript next to the formational unit abbreviation or next to the member number. For olistoliths use two lower-case letters referring to the name of the constituent unit or a Greek letter in case of igneous or lavic rocks (e.g. F° 197 "Bobbio").

The same criteria are valid also for the allostratigraphic units and unconformity bounded units, although in this case italics should be used.
Both formal and informal units should be described by means of capital letters on the sheets, unlike the databank. This choice is dictated by the need to ensure that data included on the sheets is valid for as long as possible, leaving the databank the task of making the initial distinction between formal and informal units and – following the publication of the sheets –updating information relative to the informal-formal status of the stratigraphic units on the sheet, once the Italian Stratigraphy Committee has received the worksheets.
As far as names are concerned, compilers are requested to follow the guidelines of the International Stratigraphic Guide (described in Quaderno, Serie III, n. 1 – p. 27); in this way formally defined units will be distinguished by capitalised initials while informal units will be described by lower-case initials.
For quaternary covers, reference should be made to the "dictionary of quaternary units" included in table 1, p. 19, of Quaderno, Serie III, n. 6. The order of the legend should bear in mind the different ages of the deposits (youngest followed by oldest); it must follow the stratigraphic order and not group deposits according to "type".