Patrick L. Abbott

Thermal evolution of Monte Blanco dome: Low-angle normal faulting during Gulf of California rifting and late Eocene denudation of the eastern Peninsular Ranges

Footwall rocks of the Canada David detachment fault, northeastern Baja California, record late Eocene-earliest Oligocene and late Neogene cooling events previously unrecognized in the region. Biotite ^(40)Ar/^(39)Ar ages of ∼65 Ma reflect slow cooling through ∼350°C, 5 to 10 m.y. later than is typical in the region. Multiple diffusion domain modeling of K feldspar ^(40)Ar/^(39)Ar release spectra shows very slow cooling (∼1°C/m.y.) from ∼65 to ∼45 Ma. Accelerated cooling from ∼315°C to ∼215°C between ∼45 and ∼33 Ma records at least 3–4 km of denudation that we relate to east-side-up reactivation of late Cretaceous structures that generally follow the oceanic-continental suture. Previously established rivers flowing west from mainland Mexico apparently became further entrenched during this uplift and continued to supply distinctive rhyolitic detritus to the coast. Ultimately, surface uplift disrupted and rerouted the extraregional rivers some 2 to 6 m.y. before the cooling event ended. Footwall rocks remained nearly isothermal from ∼30 to ∼15–10 Ma, when renewed rapid cooling (33° ± 17°C/m.y.) began in response to footwall exhumation by top-to-the-west low-angle normal faulting that accommodated rift-related extension in what finally became the Gulf of California. Apatite fission track and (U-Th)/He ages of ∼5 Ma and ∼4 Ma, respectively, record final detachment-related cooling through ∼110°C to ∼70°C. Thermal-kinematic modeling suggests that 5–7 km of late Neogene tectonic denudation and 10–12 km of horizontal extension were necessary in order to unroof the samples by 2 Ma. Additional extension, of unknown magnitude, has probably occurred subsequently. Geodetically measured horizontal extension rates are considerably higher than the long-term extension rate that can be attributed to detachment faulting.

Sonora, Mexico, source for the Eocene Poway Conglomerate of southern California

Alluvial-fan conglomerates of the Eocene Poway Group are composed largely of exotic rhyolite and dacite clasts derived from far to the east of their Eocene depositional site. Remnants of the Upper Jurassic bedrock source of the Poway rhyolite clasts may yet be exposed in hills in Sonora, Mexico. For this study, pieces of bedrock were taken from hills 13 km west of El Plomo in Sonora. Clasts texturally and mineralogically similar to the Sonoran bedrock were collected from the apex of the Eocene alluvial fan in San Diego County, California Nine couplets of bedrock and conglomerate clast samples (textural twins) were analyzed for 16 trace elements selected for their wide range of behaviors during magmatic and alteration processes. Statistical comparisons of the trace-element data, by using the standard error-of-the-difference method, show that there are no significant differences between the two populations. These data strongly suggest that the rhyolitic bedrock hills west of El Plomo were part of the source terrane for the Eocene conglomerate in San Diego. The latitudinal separation between bedrock source and the site of deposition is only the 2° created by the opening of the Gulf of California This implies that any boundary separating a paleomagnetically efined, Baja-Borderland terrane from the craton since Eocene time was at least 100 km east of the Gulf of California in northernmost Sonora.

On the hydrology of the Edwards Limestone, south-central Texas

Abstract The Edwards Limestone (Albian) is a mosaic of shallow water, back-reef, carbonate lithofacies averaging about 150 m thick, that have been dolomitized, chertified, and calcitized. Intermittent subaerial exposure during and shortly after deposition of the Edwards resulted in solution-enlargement of some primary voids. Slow upwarping of the northwestern margin of the subsiding ancestral Gulf of Mexico basin elevated the Edwards Group above sea level late in the Cretaceous. Down-to-the-coast, en echelon , normal faulting along the Balcones system during the Early Miocene accentuated the topographic position of the Edwards above sea level. Rejuvenated Gulf-ward flowing streams cut into the upthrown fault block and exposed the uppermost Edwards Limestone in the bottoms of deep canyons, which created the discharge sites that initiated a continuously circulating groundwater system within the Edwards Limestone in the Miocene. The early porosity systems have increased in volume through the self-ramifying cavern solution process that occurs in carbonate rocks. The down-dip boundary of the aquifer, or “bad-water line”, represents an early bypass boundary of the aquifer that has become deeply ingrained with time. Cavern development is largely sub-horizontal and reflects the control of bedding partings and beds of differing lithology. The Edwards Limestone aquifer has only a few widely separated natural discharge sites which, when viewed in scale, make the confined aquifer appear like a regional master conduit. This cavernous system is known as the Edwards underground reservoir and it supplies the water for most of south-central Texas.

Effects of Abrasion Durability on Conglomerate Clast Populations: Examples from Cretaceous and Eocene Conglomerates of the San Diego Area, California

ABSTRACT A variety of rock types subjected to abrasion durability tests in a tumbling mill yielded variable attrition rates. An abrasion durability scale derived from experiments with monolithologic pebble populations was subdivided into four groups: weakly durable marble and schist; moderately durable basalt, granodiorite, gneiss and gabbro; durable obsidian, metasandstone and metabreccia; and ultradurable silicic rhyolite, quartzite, and chert. In experiments with polylithologic pebble populations the relative durabilities remained the same but attrition rates of less durable rock types increased whereas rates of more durable types decreased. The longer the tumbling time (or travel distance), the greater the concentration of high durability clasts. Further, the presence of relatively low durability clasts increased the distance of transport necessary to achieve a given attritional loss for high durability clasts. Upper Cretaceous and Eocene conglomerates of the San Diego area are understood more clearly using these experiment-derived principles. Conglomerates of both ages were derived from the east but the Cretaceous clast populations are comprised of Peninsular Ranges detritus whereas Eocene clast populations are nearly 3/4 silicic rhyolite of exotic origin. Cretaceous conglomerates contain rock types that reflect outcrops in the nearby Peninsular Ranges; clasts vary from weakly durable to durable. The mixed durability clast suite, together with representation of all major Peninsular Ranges bedrock exposures, indicates short-distance fluvial transport. Eocene conglomerates are dominated by ultradurable rhyolite clasts that do not occur in bedrock outcrops in the Peninsular Ranges. Evidently the rhyolite clasts were introduced to a major, west-flowing fluvial system possibly originating in the modern-day area of north-central Sonora. The attrition rates of the ultradurable rhyolite clasts were decreased at the expense of the less durable, but more locally derived, Peninsular Ranges clast suite to produce conglomerates in the San Diego area that are overwhelmingly dominated by exotic rhyolite clasts, although the rhyolite source rocks probably constituted only a small fraction of the total source terrane.

A method for evaluating basement exhumation histories from closure age distributions of detrital minerals

We have developed a two-dimensional, thermokinetic model that predicts the closure age distributions of detrital minerals from pervasively intruded and differentially exhumed basement. Using this model, we outline a method to determine the denudation history of orogenic regions on the basis of closure age distributions in synorogenic to postorogenic forearc strata. At relatively high mean denudation rates of 0.5 km m.y.-1 sustained over millions of years, magmatic heating events have minimal influence upon the age distributions of detrital minerals such as K-feldspar that are moderately retentive of radiogenic Ar. At lower rates, however, the effects of batholith emplacement may be substantial. We have applied the approach to detrital K-feldspars from forearc strata derived from the deeply denuded Peninsular Ranges batholith (PRB). Agreement of the denudation history deduced from the detrital K-feldspar data with thermochronologic constraints from exposed PRB basement lead us to conclude that exhumation histories of magmatic arcs should be decipherable solely from closure age distributions of detrital minerals whose depositional age is known. (c) 1999 American Geophysical Union.

Paleogeography and Sedimentology of Upper Cretaceous Turbidites, San Diego, California

Upper Cretaceous (Campanian and Maestrichtian) marine strata of the Rosario Group in the San Diego area include the Point Loma Formation and overlying Cabrillo Formation. These units contain six facies associations: (1) shelf and lagoonal sandstone, (2) slope and basin-plain(?) mudstone, (3) outer-fan lobe sandstone, (4) middle-fan channel-fill sandstone, (5) middle-and inner-fan interchannel and channel-margin thin-bedded turbidites and mudstone, and (6) inner-fan channel-fill conglomerate and sandstone. The facies associations define a deep-sea fan deposited by westward-flowing sediment gravity flows that transported sediments derived chiefly from batholithic and prebatholithic metamorphic rocks of the Peninsular Ranges. The sedimentary basin initially deepened abruptly, partly aided by eustatic sea-level rise. The fan then prograded westward into the basin, with a retrogradational phase recorded in the uppermost part of the sequence, which is erosionally truncated by transgressive lower Eocene conglomerate. The fan was deposited along the eastern edge of a forearc basin similar to that of the Great Valley sequence in northern California. The western part of the fan, which probably contained mostly outer-fan lobe and associated basin-plain deposits, appears to have been truncated by late Cenozoic strike-slip faulting associated with the San Andreas fault system. The fan and remnants of the western part of the basin and associated subduction complex may be present on the northwest in the Channel Islands region or still farther north.

Mid-eocene climatic change, Southwestern California and Northwestern Baja California

Abstract A widespread paleosol of Paleocene and Early Eocene age occurs in southwestern California and northwestern Baja California. The dominant quartz-kaolinite mineralogy and cation-depleted chemistry of the buried soil indicate a humid, tropical paleoclimate similar to the modern equatorial belt. Although the Paleocene—Eocene paleomagnetic latitudes are similar to the modern latitudes (36–37°N to 32–33°N, respectively), rainfall was about 125–190 cm per year and average annual temperature was about 20–25°C in marked contrast to the present annual rainfall of 25 cm and average annual temperature of 16°C. A variety of indicators in the Late Eocene sedimentary succession suggests a change to a semi-arid paleoclimate. The nonmarine portions of the Late Eocene sedimentary record are dominated by a cobble conglomerate lithosome deposited in fluvial, alluvial fan and fan delta systems. Intertongued with the conglomerate is a sandstone lithosome deposited in flood-plain and nearshore marine environments. The conglomerate clasts were transported to the depositional site via a long-distance (200–300 km), moderate gradient, braided river system mostly by flash floods. Characteristic post-depositional, in situ fracturing of conglomerate clasts probably occurred due to salt crystallization. Within the flood plain sandstones, and to a lesser degree the conglomerates, are multiple well-developed caliche horizons of probable pedogenic origin. Clay minerals from the Late Eocene rocks are dominantly vermiculite and smectite with lesser chlorite and illite; this is in marked contrast to the kaolinite that comprises the underlying Early Paleogene lateritic paleosol. The character of the Late Eocene sedimentary succession indicates a semi-arid climate. Rainfall probably did not exceed 63 cm per year; it probably was seasonal and by occasional flash floods. This paleoclimate contrasts markedly with the earlier humid tropical paleoclimate and must indicate a widespread climatic change in late Middle Eocene time.

Pre-Eocene paleosol south of Tijuana, Baja California, Mexico

ABSTRACT In the Rancho Delicias area south of Tijuana, Baja California, well preserved remnants of an ancient weathering profile are developed on mid-Cretaceous granodioritic rocks. The paleosol is buried beneath Middle Eocene marine sedimentary rocks of the Delicias Formation. It has a 16 m thick C horizon characterized by grus decomposed to varying degrees. The A horizon is a severely leached oxisol that exceeds 13 m thickness and consists of approximately [2]/[3] kaolinite and [1]/[3] residual quartz gains with minor iron oxide concretions near the top. Thick latosolic soil profiles dominated by kaolinite form today only in near equatorial areas which have around 25° C average annual temperature and greater than 1,200 mm yearly rainfall. The Late Cretaceous to Early Eocene climate of th San Diego-Tijuana region was probably like that of the hot and humid areas found within 20 to 30° of the modern equator, and very unlike the present temperate, arid climate.

Paleohydrology of the eocene ballena gravels, San Diego County, California

Abstract The Ballena Gravels are remnants of a river system that flowed westward across the ancestral Peninsular Ranges during medial and late Eocene time. The Gravels (actually conglomerate) are channelized fluvial deposits that built westward as alluvial fan (Poway Group), submarine canyon (Scripps Formation) and submarine fan (Jolla Vieja Formation) depositional systems. Because the integrated sedimentary system contains distinctive Poway rhyolite clasts of limited geographic and temporal extent the now separated component formations are recognizable on the San Diego coastal plain and on the Channel Islands. Paleogeographic reconstructions suggest a transport distance of about 315 km. Multiple techniques analysis suggests the channel gradient in the San Diego area was 12–18 m km−1. Stream velocity, based on a competent particle size of 52 cm, ranges from 2.5 to 4 m s−1. Eight equations based on slope and velocity generated estimates of channel depth at food stage that vary from 2.5 to 4.5 m. Paleodischarge can be estimated from regime-type engineering equations that are based on gradient, depth, and grain size. Paleodischarge also is calculated using a technique based on stream length and drainage area. Values generated by multiple techniques suggest 2.33-yr flood discharges of about 275 m3 s−1. Extrapolation based on a logarithmic curve indicates 100-yr flood discharges around 27,500 m3 s−1. Channel-width estimates for 1 to 2.33 yr floods range from 25 to 75 m. Calculations of seasonally dominant rainfall, based on runoff and temperature, vary between 50–75 cm annually. Runoff is estimated from discharge values and paleotemperatures are based on caliche type, salt-fractured clasts, and the immature clay mineral suite. The Nueces River of Texas shares some of the same characteristics of the Eocene Ballena river. The Nueces has a highly varied discharge due to seasonally intense rainfall similar to that interpreted for the Ballena river. Several rivers flowing to the Atlantic Ocean off the eastern side of the Andes Mountains in southern Argentina may also represent modern analogues.

Rhyolite clast populations and tectonics in the California continental borderland

ABSTRACT Exotic, purple-red rhyolitic clasts are abundant constituents in upper Paleocene and Eocene conglomerates in the California Continental Borderland. Four families of rhyolitic rocks have been recognized. Owl Creek clasts have low percentages of phenocrysts that never include quartz; they are primarily found in northern localities such as the Santa Ana Mountains. Poway rhyolites are packed with phenocrysts and invariably include quartz; they dominate conglomerates in the San Diego and northern Channel Islands areas. Las Palmas clasts have a distinctive micropore-microgranular groundmass; they abound in Baja California strata. Black Rhyodacites are much darker and more brittle and were derived from a closer source terrane of bedrock and Upper Cretaceous conglomerates; they are most abund nt in the earlier deposited conglomerates of the San Diego and northern Channel Islands areas. The striking similarities between San Diego and northern Channel Islands conglomerates demonstrate that they were once part of an integrated, east-west-oriented depositional system that acts as a piercing point to document major post-Eocene strike slip in the offshore California Continental Borderland. The sudden appearance of exotic rhyolitic clasts in mixed suites in upper Paleocene conglomerates, and the subsequent increases in their abundance and decreases in their variety testify to the growth of fewer, longer, and less overlapping rivers through the Eocene. The requisite integration of fluvial drainages followed the eastward-moving front of the Laramide Orogeny, caused by a shallowing ang e of Farallon Plate subduction.