Surficial geologic map database of the Durango 1-degree by 2-degree quadrangle, southern Colorado: Contributions to the National Geologic Map

This data release presents geologic map data for the surficial geology of the Durango 1-degree by 2-degree quadrangle and adjacent areas along the northern boundary of the quadrangle. The map area lies within two physiographic provinces of Fenneman (1928): the Southern Rocky Mountains province, and the Colorado Plateau province, Navajo section. Geologic mapping is mostly compiled from published geologic map data sources ranging from 1:24,000 to 1:250,000 scale, with limited new interpretive contributions. Gaps in map compilation are related to a lack of published geologic mapping, and not necessarily a lack of surficial deposits. Much of the geology incorporated from published geologic maps is adjusted based on digital elevation model and natural-color image data sources to improve spatial resolution of the data. Spatial adjustments and new interpretations also eliminate mismatches at source map boundaries.

This data set represents only the surficial geology, defined as generally unconsolidated to moderately consolidated sedimentary deposits that are Quaternary or partly Quaternary in age, and faults that have documented Quaternary offset. Bedrock and sedimentary material directly deposited as a result of volcanic activity are not included in this database, nor are faults that are not known to have moved during the Quaternary.

Map units in the Durango quadrangle include alluvium, glacial, eolian, mass-wasting, colluvium, alluvium/colluvium, playa/lake/wetland, and groundwater/spring discharge deposit types. Alluvium map units, present throughout the map area, range in age from Quaternary-Tertiary to Holocene and form stream-channel, floodplain, terrace, alluvial-fan, and pediment deposits. Along glaciated drainages terraces are commonly made up of glacial outwash. Glacial map units are mostly middle, late middle, and late Pleistocene till likely deposited in mountain valleys during pre-Bull Lake, Bull Lake, and Pinedale glaciations, respectively; and late Pleistocene and Holocene rock-glacier deposits commonly occupying cirques in the high mountains. Eolian map units are mostly middle Pleistocene to Holocene eolian sand deposits forming sand sheets and dunes, and loess deposits forming a veneer over other surficial deposits or bedrock. Mass-wasting map units include deposits formed primarily by slide, slump, earthflow, and rock-fall or avalanche processes. Colluvium and alluvium/colluvium map units form hillslope and undifferentiated valley floor/hillslope deposits, respectively. Playa/lake/wetland map units mostly include small deposits in the San Luis Valley and in some of the glaciated valleys in the mountains. There is only one groundwater/spring discharge deposit mapped within the Durango quadrangle, a tufa deposit located in the glaciated region of the Animas River valley.

The detail of geologic mapping varies from about 1:50,000- to 1:250,000-scale depending on the scale of published geologic maps, and for new mapping, the resolution of geologic features on basemap data. Map units are organized within geologic provinces as described by the Seamless Integrated Geologic Mapping (SIGMa) (Turner and others, 2022) extension to the Geologic Map Schema (GeMS) (USGS, 2020). For this data release, first order geologic provinces are the physiographic provinces of Fenneman (1928), which reflect the major geomorphological setting affecting depositional processes. Second order provinces are physiographic sections of Fenneman (1928). Third and fourth order provinces are defined by deposit type.

Attributes derived from published source maps are recorded in the map unit polygons to preserve detail and allow database users the flexibility to create derivative map units. Map units constructed by the authors are based on geologic province, general deposit type and generalized groupings of minimum and maximum age to create a number of units typical for geologic maps of this scale.

Polygons representing map units were assigned a host of attributes to make that geology easily searchable. Each polygon contains a general depositional process (‘DepositGeneral’) as well as three fields that describe more detailed depositional processes responsible for some deposition in that polygon (‘LocalGeneticType1’ – ‘LocalGeneticType3’). Three fields describe the materials that make up the deposit (‘LocalMaterial1’ – ‘LocalMaterial3’) and the minimum and maximum chronostratigraphic age of a deposit is stored in the ‘LocalAgeMin’ and ‘LocalAgeMax’ fields, respectively. Where a polygon is associated with a prominent landform or a formal stratigraphic name the ‘LocalLandform’ and ‘LocalStratName’ fields are populated. The field ‘LocalThickness’ provides a textual summary of how thick a source publication described a deposit to be. Where three fields are used to describe the contents of a deposit, we attempt to place descriptors in a relative ordering such that the first field is most prominent, however for remotely interpreted deposits and some sources that provide generalized descriptions this was not possible. Values within these searchable fields are generally taken directly from source maps, however we do perform some conservative adjustments of values based on observations from the landscape and/or adjacent source maps. Where new features were interpreted from remote observations, we derive polygon attributes based on a conservative correlation to neighboring maps.

Detail provided at the polygon level is simplified into a map unit by matching its values to the DescriptionOfMapUnits table. Specifically, we construct map units within each province based on values of ‘DepositGeneral’ and a set of chronostratigraphic age bins that attempt to capture important aspects of Quaternary landscape evolution. Polygons are assigned to the mapunit with a corresponding ‘DepositGeneral’ and the narrowest chronostratigraphic age bin that entirely contains the ‘LocalAgeMin’ and ‘LocalAgeMax’ values of that polygon. Therefore, users may notice some mismatch between the age range of a polygon and the age range of the assigned map unit, where ‘LocalAgeMin’ and ‘LocalAgeMax’ (e.g., Holocene – Holocene) may define a shorter temporal range than suggested by the map unit (e.g., Holocene – late Pleistocene). This apparent discrepancy allows for detailed information to be preserved in the polygons, while also allowing for an integrated suite of map units that facilitate visualization over a large region.

References Cited