Fan deltas have been defined as alluvial fans that prograde into a standing body of water from an adjacent highland. Most large Holocene fan deltas are located at the edge of active continental margins and island-arc systems where high-gradient braided streams deposit their sediment load of coarse detritus. Such coastlines are usually wave dominated and receive between 100 and 300 cm of annual precipitation. The Yallahs fan delta along the southeast coast of Jamaica is a typical example of this type of fan-delta complex. The Yallahs River debouches from the Blue Mountains and has built a 10.5-sq km lobate fan delta composed of sand to boulder-size detritus. The morphology of the Yallahs delta is controlled in part by the foothills of the Blue Mountains which bound the delta on three sides and by a steep offshore profile which causes waves to break and expend most of their energy directly against the delta front. Environments comprising the subaerial delta plain include erosional and depositional beaches, abandoned and braided channels, flood plains, and salt-water ponds. The subaqueous delta environment is characterized by steep slopes and numerous submarine channels dominated by mass-gra ity processes resulting in patchy sediment-size distribution. Active coral growth occurs along the western delta margin. Sedimentologic characteristics of the Yallahs fan delta and published information on the southeast coast of Alaska provide data for the construction of two stratigraphic models that can be used for the recognition of ancient fan-delta deposits. The model based on the characteristics of the Yallahs fan delta is applicable to ancient fan-delta deposits that grade directly onto continental and/or island-arc slopes. These deposits are characterized by proximal, gravelly, braided-stream deposits that grade seaward into interbedded gravels and sands at the shoreline and to muddy gravels and muds of the slope. The second model, based on the characteristics of the southeast Alaska coastline, is applicable to fan-delta deposits that prograde onto continental and/or island shelves. These deposi s are characterized by gravelly, proximal, braided-stream deposits that grade seaward into sandy, distal braided-stream deposits, well-laminated sands of the beach and nearshore zone, and finally into burrowed shoreface muds. The two models represent end members of a spectrum of fan-delta types common in ancient sedimentary sequences ranging in age from Precambrian to Pleistocene.

Fan-Delta Sedimentology and Tectonic Setting--Yallahs Fan Delta, Southeast Jamaica

Stromatolites are abundant in shallow marine sediments deposited before the evolution of animals, but in the modern ocean they are restricted to locations where the activity of animals is limited. Overall decline in the abundance of stromatolites has, therefore, been attributed to the evolution of substrate-modifying metazoans, with Phanerozoic stromatolite resurgences attributed to the aftermaths of mass extinctions. Here we use a comprehensive stratigraphic database, the published literature, and a machine reading system to show that the rock record–normalized occurrence of stromatolites in marine environments in North America exhibits three phases: an initial Paleoproterozoic (ca. 2500 Ma) increase, a sustained interval of dominance during the Proterozoic (2500–800 Ma), and a late Neoproterozoic (700–541 Ma) decline to lower mean prevalence during the Phanerozoic (541–0 Ma). Stromatolites continued to exhibit large changes in prevalence after the evolution of metazoans, and they transiently achieved Proterozoic-like prevalence during the Paleozoic. The aftermaths of major mass extinctions are not well correlated with stromatolite resurgence. Instead, stromatolite occurrence is well predicted by the prevalence of dolomite, a shift in carbonate mineralogy that is sensitive to changes in water-column and pore-water chemistry occurring during continent-scale marine transgressive-regressive cycles.

The rise and fall of stromatolites in shallow marine environments

Cambrian sandstones of the western United States and northern Mexico record deposition above a major unconformity on the rift margin of Laurentia. Detrital zircon ages from the Cambrian-Ordovician Bliss Sandstone in southern New Mexico are used to test models for the influence and significance of the Transcontinental Arch during the early Paleozoic. We obtained U-Pb zircon ages of basement rocks to test potential local source rocks in New Mexico. An orthogneiss in the Florida Mountains has an age of 1627 ± 18 Ma (all errors 2σ). The San Diego Mountain granite underlying the Bliss Sandstone is ∼1400 Ma. A rapakivi granite in the Little Hatchet Mountains has a U-Pb Concordia age of 1077 ± 4 Ma, coeval with the Pikes Peak granite and several granites in Sonora, Mexico. Three samples of the Florida Mountains granite underlying the Bliss Sandstone yield an average age of 510 ± 5 Ma; this is a rare and distinctive age of magmatism in western Laurentia. Zircons with these ages can be used as tracers for understanding transportation paths of sediment. The Bliss Sandstone was studied from eight localities in southern New Mexico; one overlies the Cambrian Florida Mountains granite, and three lie to the west. The sandstones are quartzarenite, subarkose, and arkose. The most abundant detrital zircon population included Grenville ages with a main peak at 1250 Ma. Other significant peaks are at 1465 Ma, 1625 Ma, 1710 Ma, and 1850 Ma, and these populations generally decrease in abundance with increasing age. Only five Archean zircons (out of 845) were discovered. Cambrian detrital zircons are present in the Florida Mountains Bliss Sandstone locality, directly overlying the Cambrian granite. They are also present in two localities 60–80 km southwest and northwest of the Florida Mountains. Maximum depositional ages from sandstones with Cambrian zircons range from 509 ± 7 Ma to 504 ± 12 Ma, overlapping with the age of the Florida Mountains granite. The four localities east of the Florida Mountains did not have any Cambrian detrital zircons. We interpret these data as indicating that the Cambrian intrusions of the Oklahoma-Colorado rift were not a source for the Cambrian zircons in the Bliss Sandstone. The Tapeats Sandstone in the Grand Canyon does not have Cambrian zircons and has a minor Grenville peak at 1070 Ma (Gehrels et al., 2011), significantly younger than the 1250 Ma Grenville peak in the Bliss Sandstone. We interpret these differences as being related to different source areas on opposite sides of the Transcontinental Arch. The differences in the specific ages of Cambrian plutons between the Oklahoma rift and the Florida Mountains granite can be used to test depositional models for other Cambrian sandstones of the southwest U.S. and in Mexico, where the presence of Cambrian zircons in the Caborca Block has implications for the Mojave-Sonora megashear hypothesis.

Detrital zircon geochronology from the Cambrian-Ordovician Bliss Sandstone, New Mexico: Evidence for contrasting Grenville-age and Cambrian sources on opposite sides of the Transcontinental Arch

Left-lateral intraplate deformation along the ancestral rocky mountains: Implications for late paleozoic plate motions

Both from an ecological and geomorphological stand point, studies on modern rocky shores are common place in the scientific literature. Published studies on ancient rocky shores through geologic time are still very rare by comparison. Review of the literature, however, shows the topic to be a fast growing discipline of potentially broad interest. The 155 research articles which comprise this annotated bibliography illustrate many different applications for data from ancient rocky shores. They include: paleogeographic mapping, bench mark for associated paleontological and sedimentological zonations, calculation of coastal uplift or subsidence rates, test for eustatic sea-level changes, comparative geomorphology of ancient rocky coasts, and development of the paleobiology and evolutionary history of rocky-shore biotas. One of the earliest thinkers about ancient rocky shores was Benoit de Maillet, whose observations were published posthumously in 1748. During the late 18th, the 19th, and the early 20th centuries, work was refocused on the concept of geological unconformities. Major advancements were made by Antoine Lavoisier, James Hutton, John Playfair, Henry De La Beche, Andrew Ramsay and Amadeus Grabau. The first journal articles devoted to analysis of specific geological sites did not appear until after the turn of the 20th century. From 1905 to 1954, research papers were published sporadically at an average rate of 0.3/year. From 1955 to 1979, the flow of research quickened, with an average rate of 2.2 papers/ year. The following five-year period 1980-1984 saw a doubling in rate to 4.6 papers/year. The rate doubled again to 9.4 papers/year during 1985-1989. Thus, the three consecutive half decades from 1975 to 1989 witnessed exponential growth in terms of published research. Examples of rocky shores from all geological periods in the Phanerozoic are represented by articles in the bibliography; the oldest known rocky shore from the Precambrian is 3.3-3.5 billion years old.

Studies on Ancient Rocky Shores: A Brief History and Annotated Bibliography

A large northwest-trending graben-faulted anticline involving all pre-Pennsylvanian rocks is exposed in the core of the San Juan Mountains near Silverton, Colorado. The feature was formed before Late Cambrian time when younger Precambrian quartzites were extensively down-faulted into juxtaposition with the older Precambrian basement complex. The quartzites emerged topographically high and supplied local talus to Ignacio deposits (Late Cambrian) adjacent to the faults. Ignacio rocks do not cover the fault block, but thicken and become finer-grained away from the feature. As the Late Devonian seas advanced, the McCracken Sandstone Member of the Elbert Formation was deposited along the flanks of the structural feature, but apparently did not cross it. The upper Elbert intertidal dolomites were the first sediments to be deposited across the faulted fold where they lie directly on Precambrian quartzites. Latest Devonian or earliest Mississippian stromatolitic dolomites of the Ouray Formation overlie Elbert strata on the flanks of the old feature, and equivalent normal marine limestones are locally present within the down-faulted block. The Early Mississippian Leadville Formation thins abruptly onto the flanks of the graben-faulted anticline, where the lithologic aspect is one of stromatolitic dolomite formed in an inter-tidal zone. Within the graben, Leadville strata are thin or missing except as remnants in the overlying Molas regolith, suggesting that intense post-Leadville weathering removed those rocks across the regionally high structure. A small horst is present within the large graben where Pennsylvanian sedimentary rocks rest directly on upturned Precambrian quartzites. The graben was again down-faulted at some undetermined post-Desmoinesian time, for Middle Pennsylvanian beds are now in fault contact with Precambrian rocks along the flanking faults. A similar ancient fault block south of Ouray, Colorado, extends northwesterly into the sub-surface of the eastern Paradox basin. This feature parallels the nearby Uncompahgre uplift and may be genetically related to it.

Paul D See

Papers

Pre-Pennsylvanian Stratigraphy and Paleotectonics of the San Juan Mountains, Southwestern Colorado