SEQUENCE STRATIGRAPHIC ANALYSIS OF THE BASAL SAUK SEQUENCE ACROSS A CRATON HINGE ZONE, SOUTHEASTERN CALIFORNIA

In SE California, the basal unit of the Sauk Sequence is represented by Lower Cambrian feldspathic sandstones of the middle member of the Wood Canyon Formation (mmWCF), which rests nonconformably on incised Mesoproterozoic crystalline basement. The basal contact becomes apparently more conformable with underlying strata in a basinward direction, where the mmWCF rests on a northwest thickening wedge of Neoproterozoic strata, including shallow marine shales and shoaling bank carbonates of the lower member of the Wood Canyon Formation (lmWCF), prompting questioning of the hiatal magnitude in more offshore settings. Correlation of a distinct conglomerate in the lower part of the mmWCF across the craton edge is crucial to determining whether the contact has significant regional erosive impact because such a unit is potentially consistent with incised valley fill. Stratigraphically, the conglomerate occurs at the base of the mmWCF in some locations, rests meters above the base as one or multiple distinct units in other locations, or, in some cases, is not present at all. Clast counts in the field were performed using a 1 m by 1 m grid with 5 cm increments. Clast grain size distributions range from granules to small cobbles with an average phi size of -3.9 and -3.4 (medium pebble) for cratonic and miogeoclinal sections, respectively. At each location, full 3D parameters of clasts were measured because they had weathered out of the outcrop. All populations have average sphericities of 0.73 ± 0.01. Comparisons of long axis measurements, using both in situ and loose gravel data, reveal similar distributions from all locations. Compositionally, the dominant clast type is vein quartz, ranging from 43% to 53% in cratonic, and up to 77% in miogeoclinal sections. Chert and chalcedony clasts make up 19% to 28% in cratonic settings and 4% to 12% in the miogeocline. Jasper is present at less than 6% in all locations. Although very similar in composition, data obtained thus far does not require that the mmWCF conglomerate is correlative across the craton margin. Further stratigraphic and textural work on more examples of the conglomerate is necessary. If the mmWCF conglomerate is not responsible for significant truncation of the lmWCF, then perhaps one or multiple sequence boundaries are concealed within the shale and/or carbonate units of the lmWCF.