Geology and Ceramic Properties of the Ione Formation, Buena Vista Area

This collaborative research project was conducted for the purpose of assessing the refractory (ceramic) clay reserves in an area immediately to the south of the principle clay mining region surrounding the town of Ione, California in Amador County.  Joseph Pask was a ceramic engineer and Mort Turner was a geologist with the California Division of Mines and Geology—the predecessor of the California Geological Survey.

The primary analytical tool used to characterize the clay materials in this study was Differential Thermal Analysis (DTA) which is/was used in the ceramics industry to define the firing properties and suitability of clays for using in ceramics manufacture.  DTA analysis does not define the mineralogy of the clay mineral assemblage such as the methods used today, i.e., clay mineral x-ray diffraction analysis (XRD).  However, the DTA results posted in this study did give semi-qualitative clay mineral indications based on the DTA curves.  For example, the authors did determine that the clay mineral assemblage of the geologic unit they called the “upper Ione member” had a substantial smectite clay component.  The authors differentiated the “lower Ione” member clay mineral assemblage as being dominated by kaolinite clay similarly based on the DTA analyses.

The authors also recognized that the “upper Ione” sandstone unit has up to 25% feldspar in the sand fraction compared to the largely quartzose sand in the lower Ione unit.  These two factors, ie., greater feldspar component and smectitic mineralogy, collectively suggested to the authors that the “upper Ione” sediments were subjected to a less intense chemical weathering regimen in a cooler/dryer climate compared to the purely tropical climate that prevailed during the time of the “lower Ione” sedimentation.

In addition, the authors described the finer grained clay beds (mudstones) in the upper Ione member as having a green color.

All these factors indicate that the unit that these authors called the upper Ione unit is equivalent to the mineralogy and description of the sedimentary unit to the north called the Chalk Bluff lithology.  See the GSA presentation on this topic for more details here.

The authors noted the mineralogical differences were of such contrast that they defined the upper Ione section as a unique member status.  Now that XRD techniques actually quantify Pask and Turner’s “upper Ione member” as having a unique smectitic clay mineral suite that can be traced and correlated throughout the foothills of Northern California, nomenclature rules preclude such a mineralogical difference as being classified as a submember of the Ione Fm., especially considering the distinct angular unconformity preserved between the Ione sediments and the upper smectitic sediments.  As these researchers’ mission was to qualify the clay deposits for suitability as ceramic clay reserves, they weren’t focused on the criteria for classifying and elevating the upper smectitic sediments to a unique Formation status.


Pask, Joseph A. and Mort D. Turner, 1952, Geology and Ceramic Properties of the Ione Formation, Buena Vista Area, Amador County, California; Special Report 19, Division of Mines, California Dept of Natural Resources, San Francisco.

Special Report 19:      Text        Plate 1     Plate 2      Plate 3     Plate 4