New Mexico Geological Society Annual Spring Meeting — Abstracts

Various investigators have interpreted exposures of Precambrian rocks east of Socorro as "islands" exposed during Pennsylvanian time, as basement blocks upfaulted during the development of the Rio Grande Rift Zone, or as "rootless" masses inserted into the section by shearing and thrust faulting. The Gonzales Precambrian Block is defined by two of these masses approximately one mile apart in an east-west direction; the western exposure is bounded on the west by an high-angle normal fault placing Precambrian granitic rock against Santa Fe Group rocks, and on the east by an east-dipping series of coarse-grained clastic that are assigned to the Pennsylvanian Sandia Formation. The eastern exposure is bounded on the west by an high-angle normal fault placing Pennsylvanian interbedded limestone and clastic layers against Precambrian granitic rock, and on the east by vertical to overturned clastic beds that are also assigned to the Pennsylvanian Sandia Formation. Similar relations occur with several other Precambrian masses south of the Gonzales Block.

In an attempt to resolve the interpretations mentioned above detailed mapping (Plane Table 100'=1") of the eastern Gonzales exposure has been completed. Any solution must answer several questions: First, are the Precambrian-Pennsylvanian contacts depositional or tectonic? Second, do the vertical and overturned Pennsylvanian strata east of the eastern exposure result from a basement-core overturned fold? Third, why do the northern and southern margins of the Precambrian exposures lack clear structural definition? Fourth, what is the relation between the several Precambrian masses and the overturned folds exposed in the Pennsylvanian and Permian rocks farther to the east.

Study of the western Precambrian exposure shows the Precambrian-Pennsylvanian contact is depositional and the overlying east-dipping Sandia beds expose a nearly complete section broken by several high-angle normal (?) faults. Measurement of this section shows that approximately one-half to two-thirds of the Sandia section is missing at the eastern Precambrian exposure.

Mapping of the eastern mass shows several Precambrian-Pennsylvanian contacts, all of which are tectonic and exhibit shearing. The folding may be a faulted fault-bend fold or a fault-propagation fold similar to others known to the south and east. Finally, a later thrust fault has overridden the fold and part of the Precambrian mass; a small klippe of coarse-grained sandstone correlated with the lower part of the Sandia Formation is all that remains of the thrust sheet.

Low-angle faulting, formerly thought confined to Permian strata, involves more stratigraphic section, exhibits ramp-flat geometry and is more widespread than previously believed.