A MIDDLE PLEISTOCENE ENVIRONMENTAL RECORD FROM CALIFORNIA’S GREAT LAKE CORCORAN

In the southern Great Valley hydrogeologists have mapped a distinct and extensive, 2-10 m thick clay stratum, usually at depths >40 m. An aquitard important to water-management and locally called the “E Clay,” it is formally known as the Corcoran Clay Member of the Tulare Formation (CCM). Because of its importance in groundwater management, it is recorded in well over a thousand well logs. Evidently a lake deposit, mapping out this blue-black clay shows that it was vast: >400 km long and with an area >16,000 sq km. The fossils discussed here are from sediment ca. 8 km from its reconstructed northeast margin, near Turlock and about equidistant between the Tuolumne and Merced rivers. During industrial construction there, a well-preserved walnut (Juglans cf. californica) was recovered at ca. 49 m depth. Subsequently, sediment samples were collected whenever the CCM was penetrated, and screen-washed for identifiable fossils. Plant remains that were recovered, many appearing charred, included redwood (Sequoia or Sequoiadendron), firs (Abies spp.), pines (Pinus spp.) and manzanita (Arctostaphylos spp.). Given Turlock’s aridity and distance (>50 km) from the wooded slopes of the Sierra Nevada and the Coast Range, these conifers are an anomalous occurrence. Walnut probably occupied riparian habitats along river courses, but today redwood, fir and pine occur only in the distant mountains, at much higher elevations.

An age-depth model for the CCM suggests it dates from Marine Isotope Stage (MIS) 19c to MIS 16a, or ca. 800 to 630 kya. The global stable-isotope record indicates MIS 19c was typified by interglacial conditions: redwood and manzanita are present in sediments of that apparent age. In contrast, pine and fir are present in MIS 16a, a glacial-maximum period, while redwood and manzanita are not. Many of the fossils appear charred and they may represent detritus washed into ancient Lake Corcoran by floods after wildfires and watershed deforestation. The ancestral Merced River may have been their source but, on such a vast paleolake, rafts of plant debris could have been transported far by longshore currents. Wildfire, flood, and then transport as flotsam that eventually sank explains upland tree fossils so far into the paleolake, while their variability with depth appears to provide a paleoenvironmental signal.