Merced Formation Research Articles

A Middle Pleistocene Sea Otter from Northern California and the Antiquity of Enhydra in the Pacific Basin

Extant sea otters (Enhydra lutris) are remarkably well understood in terms of behavior, ecology, and interactions with humans, but the evolutionary history of this charismatic marine mammal is limited owing to a fragmentary fossil record. Disagreements over the generic assignment of various fossil otter remains to members of the tribe Enhydrini, and limited geochronologic data for these records have impeded attempts to interpret the evolutionary biogeography of Enhydra. A well-preserved femur of Enhydra sp. from a middle Pleistocene horizon within the Merced Formation of northern California is the oldest record of Enhydra in the Pacific with robust geochronologic age control. Bracketing 87Sr/86Sr dates indicate an age of 620–670 ka. Reappraisal of the geochronologic age of various occurrences of Enhydrini indicate dispersal of Enhydra into the Pacific through the Bering Strait no earlier than the middle Pleistocene. Somewhat older early Pleistocene fossils of Enhydra from Alaska and England suggest an Arctic or North Atlantic origin of the Enhydra lineage.

Vertical tectonic deformation associated with the San Andreas fault zone offshore of San Francisco, California

A new fault map of the shelf offshore of San Francisco, California shows that faulting occurs as a distributed shear zone that involves many fault strands with the principal displacement taken up by the San Andreas fault and the eastern strand of the San Gregorio fault zone. Structures associated with the offshore faulting show compressive deformation near where the San Andreas fault goes offshore, but deformation becomes extensional several km to the north off of the Golden Gate. Our new fault map serves as the basis for a 3-D finite element model that shows that the block between the San Andreas and San Gregorio fault zone is subsiding at a long-term rate of about 0.2–0.3 mm/yr, with the maximum subsidence occurring northwest of the Golden Gate in the area of a mapped transtensional basin. Although the long-term rates of vertical displacement primarily show subsidence, the model of coseismic deformation associated with the 1906 San Francisco earthquake indicates that uplift on the order of 10–15 cm occurred in the block northeast of the San Andreas fault. Since 1906, 5–6 cm of regional subsidence has occurred in that block. One implication of our model is that the transfer of slip from the San Andreas fault to a fault 5 km to the east, the Golden Gate fault, is not required for the area offshore of San Francisco to be in extension. This has implications for both the deposition of thick Pliocene–Pleistocene sediments (the Merced Formation) observed east of the San Andreas fault, and the age of the Peninsula segment of the San Andreas fault.

The middle Pleistocene Merced-2 and -3 sequences from Ocean Beach, San Francisco

The Merced Formation comprises a 2-km-thick shallow marine and non-marine succession that was deposited in a small transtensional basin along the San Andreas Fault during the late Pliocene to middle Pleistocene. The sediments dip between 10° and 80° to the northeast, and are locally disrupted by small faults. During tilting, the beds have been rotated into subparallelism with the San Andreas Fault zone, splays of which bound the outcrop belt of Merced sediments to both the southwest and northeast. The Merced Formation contains more than 20 transgressive–regressive sedimentary rhythms (cyclothems, or sequences) that are generally between 40 and 120 m thick, and which were deposited mostly during interglacial time, under the influence of rising, highstand and early falling sea levels. Sequences Merced-2 [units M1–N of Soc. Econ. Paleontol. Mineral., Field Guidebook 3 (1984) 1] and Merced-3 (units O–P), though in fault contact, comprise typical Plio-Pleistocene shallow water cyclothems. The Merced-2 Sequence is 22+ m thick, and comprises a sandy and shelly transgressive systems tract, including a basal Type A shellbed, an in situ Type B mid-cycle shellbed, and a highstand systems tract of massive siltstone. The Merced-3 Sequence is 47 m thick, and comprises a basal compound shellbed, a thin highstand systems tract siltstone, and a sand-rich regressive systems tract. The RST comprises distal shoreface sands with an abundant in situ molluscan fauna, and upper shoreface and back-beach trough cross-bedded sands and pebbly sands. The top of the Merced-3 cycle comprises a beach sand capped by a palaeosol and lignite (the “Beetle Bed”), which together mark the subaerial exposure of the site during the ensuing glacial sea-level lowstand. Analysis of 10Be across the Merced-3 Sequence shows major peaks, indicative of sedimentation starvation, in the basal transgressive systems tract shellbed and in the capping lignite of the Beetle Bed. Smaller 10Be peaks are associated with a shellbed that is inferred to represent winnowing at the foot of the shoreface, and with a minor exposure surface that delimits a small paracycle in the top of the sequence. Otherwise, 10Be abundances decline regularly across the Merced-3 Sequence, consistent with an increasing sedimentation rate as shoreface progradation, and regression, progressed. The character of cycles Merced-2 and Merced-3 respectively resembles the Seafield and Rangitikei sequence motifs described from similar Plio-Pleistocene sediments in New Zealand. The cycles are of mid-Pleistocene age, and were probably deposited during interglacial oxygen isotope stages 21 and 19, respectively.

The San Andreas Fault in the San Francisco Bay Region, California: Structure and Kinematics of a Young Plate Boundary

Recently acquired high-resolution aeromagnetic data delineate offset and/or truncated magnetic rock bodies of the Franciscan Complex that define the location and structure of, and total offset across, the San Andreas fault in the San Francisco Bay region. Two distinctive magnetic anomalies caused by ultramafic rocks and metabasalts east of, and truncated at, the San Andreas fault have clear counterparts west of the fault that indicate a total right-lateral offset of only 22 km on the Peninsula segment, the active strand that ruptured in 1906. The location of the Peninsula segment is well defined magnetically on the northern peninsula where it goes offshore, and can be traced along strike an additional ˜6 km to the northwest. Just offshore from Lake Merced, the inferred fault trace steps right (northeast) 3 km onto a nearly parallel strand that can be traced magnetically northwest more than 20 km as the linear northeast edge of a magnetic block bounded by the San Andreas fault, the Pilarcitos fault, and the San Gregorio-Hosgri fault zone. This right-stepping strand, the Golden Gate segment, joins the eastern mapped trace of the San Andreas fault at Bolinas Lagoon and projects back onshore to the southeast near Lake Merced. Inversion of detailed gravity data on the San Francisco Peninsula reveals a 3 km wide basin situated between the two strands of the San Andreas fault, floored by Franciscan basement and filled with Plio-Quaternary sedimentary deposits of the Merced and Colma formations. The basin, ˜1 km deep at the coast, narrows and becomes thinner to the southeast along the fault over a distance of ˜12 km. The length, width, and location of the basin between the two strands are consistent with a pull-apart basin formed behind the right step in the right-lateral strike-slip San Andreas fault system and currently moving southeast with the North American plate. Slight nonparallelism of the two strands bounding the basin (implying a small component of convergence with continued strike-slip movement) may explain the progressive narrowing of the basin to the southeast and the puzzling recent uplift of the Merced Formation in a predominantly extensional (pull-apart basin) setting. The 1906 San Francisco earthquake may have nucleated within the step-over region, and the step-over places a strand of the San Andreas fault 3 km closer to downtown San Francisco than previously thought.

Strontium Isotope Ages of the Marine Merced Formation, near San Francisco, California

The strontium isotopic compositions of foraminifers (Elphidiella hannaiwere determined to refine the age of coastal sediments of the Merced Formation and to evaluate the relation between transgressive–regressive cycles and Pleistocene glacio-eustatic sea-level fluctuations. Relatively good age resolution (± 0.1 myr) is possible in the upper part of the section. The Sr isotope ages provide strong supporting evidence for the fission track ages of an ash layer at the top of the formation (0.45 myr), as well as recent age estimates for the mammoth fossil of ∼0.45 myr. Sr isotope ages in the upper part of the section also confirm that marine/nonmarine cyclic sedimentation in the Merced Formation is controlled by glacial–interglacial cycles. Good age resolution for the lower part of the section is not possible, due to little variation of the87Sr/86Sr ratio in global sea water between 2.4 and 4.3 myr. The age of the base of the formation is constrained only between 2.4 and 4.8 myr. Sediment accumulation rates in the upper portion of the Merced Formation (between 0 and 720 m) are significantly higher than those calculated for the basal portion (between 720 and 1670 m). The change in accumulation rate may be due to increased continental weathering rates and/or increased tectonic subsidence beginning about 0.8 myr ago.

Sedimentologic Approaches to Paleobathymetry, with Applications to the Merced Formation of Central California

Sedimentologic evidence for paleobathymetty, like paleontologic evidence, is largely indirect in that it mostly reflects processes and conditions that are themselves partly depth-dependent. Sedimentologic approaches can be a useful adjunct to paleontology for interpreting paleodepths, particularly in relatively shallow water. Pleistocene deposits of the Merced Formation on the San Francisco Peninsula, California, provide an excellent opportunity to demonstrate the methods, value, and difficulties of the sedimentologic approach. Distinction between subaerial and subaqueous deposition can be drawn on the basis of eolian structure, paleosols, and other sedimentary features. The spacing of symmetric ripples relative to their grain size and lenticular laminae of heavy minerals may be useful for identifying deposition in very shallow water. The boundary between intertidal and subtidal deposits serves to establish ancient sea-level positions. The boundary is reflected by the upward transitionsfrom cross-bedded sand (nearshore facies) to planar-bedded sand (foreshore facies) in open-coast deposits, and from laminated sand andlor mud (subtidal channel bank facies) to bioturbated sand andlor mud (intertidal flat facies) in the deposits of coastal embayments. Where the physical setting (i.e., wave energy, sediment input, etc.) can be reasonably reconstructed, paleodepths in shelffacies are indicated by the maximum depth of cross-bedding and gravel deposits generated by the effects of shoaling waves, the contact between shelf sand and mud, and the deepest occurrence of shell lags. In Pleistocene shelf deposits of the Merced Formation on the San Francisco Peninsula, California, these depths are placed at 10 + 5 m, 60 + 10 m, and 100 + 20 m, respectively, based on their inferred depth on the modern central California shelf. At greater depths the sedimentologic approach loses resolution and reliability. Interpretations based on the distribution of turbidites, evidence of anoxic conditions, and calcium carbonate compensation depths and lysoclines are probably most useful for establishing paleodepth in a relative sense only. Integration of the sedimentologic and paleontologic approaches carries a high potential for improving our ability to determine paleobathymetry. INTRODUCTION

Diagenetic Stability of Temperature-Sensitive Skeletal Properties in Mytilus from the Pleistocene of California

Skeletal structure, strontium and magnesium content, and mineralogy of the pelecypod Mytilus have been shown to vary with growth temperature and salinity. Based on these skeletal properties, paleotemperatures were determined from fossil specimens of Mytilus from five Pleistocene exposures along the Pacific coast. Paleotemperatures determined by the various methods do not always agree, suggesting that diagenesis has differentially modified some of the skeletal properties. As skeletal structure is not easily destroyed by diagenesis, skeletal-structure paleotemperatures can be used to evaluate the extent of alteration of shell chemistry and mineralogy. Strontium content appears to be altered at only one of the sites studied. A small amount of conversion of aragonite to calcite seems to have occurred. Oxygen isotopic composition was also determined for a sample from each locality. When used in conjunction with paleotemperatures determined by the other methods, oxygen isotopic composition of the samples can be used to determine the δO18 value of the water in which the shell grew, yielding information on paleoclimatic conditions. Paleotemperatures for all late Pleistocene localities are near present-day surface water temperature values for the nearby ocean. The paleotemperature data and oxygen isotope data suggest an interglacial origin for all late Pleistocene localities. The strontium content and δO18 value for the early Pleistocene (upper Merced Formation) locality indicate a lower temperature and slightly reduced salinity.