Abstract The results of drilling during Ocean Drilling Program Leg 180 provide insights into fundamental processes of continental break-up, because rifting can be related to westward propagation of a spreading centre into continental crust. A generally north–south transect of holes was drilled across the Woodlark Rift on the uplifted northern rift margin on the Moresby Seamount (Sites 1114 and 1116), on the hanging wall of the low-angle (25–30°) extensional Moresby Detachment Fault (Sites 1108, 1110–1113 and 1117) and across the downflexed northern rift margin (Sites 1118, 1109 and 1115). The results, when placed in the regional tectonic context, document a history of Palaeogene ophiolite emplacement, followed by Miocene arc-related sedimentation. Regional uplift and emergence of the forearc area took place in Late Miocene time. Submergence to form the Woodlark Rift began in latest Miocene time, marked by widespread marine transgression and shallow-water deposition, accompanied by input of air-fall tephra and volcaniclastic sediments. During Pliocene time, deposition within the rift basin was dominated by deep-water turbidites, including high-density turbidites in the south. Strong extension along the north-dipping Moresby Detachment Fault was active during Pleistocene time, associated with uplift of the Moresby Seamount and shedding of fault-derived talus, mainly of meta-ophiolitic origin. During Pleistocene time, a carbonate platform was constructed to the NW, trapping clastic sediment and resulting in a switch to slower, more pelagic and hemipelagic deposition within the Woodlark Rift Basin. The marked change in rift basin configuration during Pleistocene time may relate to westward propagation of the Woodlark oceanic spreading centre at c. 2 Ma.

We report 777 U-Pb SHRIMP detrital zircon ages from thirteen sandstones and metasandstones in interior Alaska. About sixty grains per sample were analyzed; typically, half to three-fourths of these were concordant within ± 10%. Farewell terrane . Two quartzites were collected from Ruby quadrangle and a third from Taylor Mountains quadrangle. All three are interpreted to represent a low stratigraphic level in the Nixon Fork platform succession; the samples from Ruby quadrangle are probably late Neoproterozoic, and the sample from Taylor Mountains quadrangle is probably Cambrian in age. The youngest detrital zircon in any of the three is 851 Ma. The two Ruby quadrangle samples area almost identical: one has a major age cluster at 1980–2087 and minor age clusters at 944–974 and 1366–1383 Ma; the other has a major age cluster at 1993–2095 Ma and minor age clusters at 912–946 and 1366–1395 Ma. The Taylor Mountains sample shows one dominant peak at 1914–2057 Ma. Notably absent are zircons in the range 1800–1900 Ma, which are typical of North American sources. The detrital zircon populations are consistent with paleontological evidence for a peri-Siberian position of the Farewell terrane during the early Paleozoic. Mystic subterrane of the Farewell terrane. Three graywackes from flysch of the Mystic subterrane, Talkeetna quadrangle, were sampled with the expectation that all three were Pennsylvanian. Asample from Pingston Creek is Triassic (as revealed by an interbedded ash dated at ca. 223 Ma) and is dominated by age clusters of 341–359 and 1804–1866 Ma, both consistent with a sediment source in the Yukon-Tanana terrane. Minor age clusters at 848–869 and 1992–2018 Ma could have been sourced in the older part of the Farewell terrane. Still other minor age clusters at 432–461, 620–657, 1509–1536, and 1627–1653 Ma are not readily linked to sources that are now nearby. A sample from Surprise Glacier is mid-Mississippian or younger. A dominant age cluster at 1855–1883 and a minor one at 361–367 Ma could have been sourced in the Yukon-Tanana terrane. Other age clusters at 335–336, 457–472, 510–583, and 1902–1930 have no obvious nearby source. A sample from Ripsnorter Creek is Silurian or younger. The dominant age cluster at 937–981 Ma and a minor one at 2047–2077 Ma could have been sourced in the Farewell terrane. Minor age clusters at 1885–1900 and 2719–2770 Ma could have been sourced in the Yukon-Tanana terrane. Other age clusters at 429–490, 524–555, 644–652, 1023–1057, 1131–1185, and 1436–1445 Ma have no obvious nearby source. The so-called Mystic subterrane is structurally complex and would appear to include more than one Phanerozoic turbidite succession; more mapping and detrital zircon geochronology are needed. Wickersham and Yukon-Tanana terranes. A grit from Wickersham terrane in Tanana quadrangle and a grit from Yukon-Tanana terrane in Talkeetna quadrangle have similar, exclusively Precambrian detrital zircon populations, supporting previous correlations. The Wickersham sample has major age clusters at 1776–1851 and 1930–1964 Ma, and the youngest grain is 1198 Ma. The Yukon-Tanana grit has a major age cluster at 1834–1867 Ma, and the youngest grain is 1789 Ma. A North American source has been previously proposed, and this seems likely based on detrital zircon data. Ruby terrane and Minook Complex. Detrital zircons from quartzites in the Ruby terrane show two quite different age patterns. A sample from the Bear Creek area of Tanana quadrangle has detrital zircon ages that are similar to those from the Wickersham and Yukon-Tanana grits. The dominant age clusters are 1823–1856 and 1887–1931 Ma. In contrast, a quartzite from nearby Senatis Mountain (Tanana quadrangle) yielded a completely different detrital zircon age spectrum, featuring a broad peak with no significant gaps from 1024 to 1499 Ma and a minor age cluster at 1671–1695 Ma. The youngest concordant zircon is 1024 ± 6 Ma. A quartzite from the Minook Complex, a sliver along the Victoria Creek strike-slip fault in Tanana quadrangle, is similar to the Senatis Mountain sample. Its detrital zircon population is dominated by grains between 1103 and 1499 Ma, with peaks within that range at 1161–1234 and 1410–1490 Ma; minorolderage clusters are at 1643–1676, 1765–1781, and 1840–1874 Ma. The youngest concordant grain is 1103 ± 6 Ma. Finally, a quartzite from Illinois Creek (Nulato quadrangle) at the extreme west end of the Ruby geanticline, previously assigned to the Ruby terrane, also has a detrital zircon age spectrum like that at Senatis Mountain. Mesoproterozoic zircons are predominant, with main age groups at 1329–1391 and 1439–1493 Ma and lesser ones at 1058–1072, 1184–1193, 1681–1692, and 1852–1879 Ma. The youngest concordant grain is 1058 ± 33 Ma. These barcodes are dominated by Mesoproterozoic zircons that are strikingly similar in age to detrital zircons in Neoproterozoic Sequence B in northwestern Canada (and easternmost Alaska, where it equates to the lower Tindir Group). Among other rocks, the Ruby geanticline thus might include a shortened, metamorphosed, and offset continuation of this ancient North American basin, which was sourced in the Grenville orogen. Rampart Group, Angayucham-Tozitna terrane. The Rampart Group is thought to have been deposited in an ocean basin that closed during the Brookian Orogeny. Detrital zircons from graywacke of the Rampart Group are dominated by an age cluster at 380–404 Ma, with lesser ones at 351–364, 426–440, 484–504, 909–920, 1001–1020, 1127–1128, 1211–1217, and 1912–1953 Ma. The youngest grain is 260 ± 1 Ma. The dominant 380–404 Ma age cluster can be reasonably linked to sources in Devonian plutons of the now-adjacent Brooks Range and Ruby terrane.

Abstract In the early days of Gulf of Mexico piston coring, locations were chosen on a grid basis or selected from loose 2D seismic surveys. Such locations resulted in some seepage “hits,” but the majority had either a background signature or an “anomalous” value that was between a true visible seep and background (using fluorescence intensity and unresolved complex mixture content). A scale based on these early data identified anything <5,000-10,000 fluorescence units as background, and those values >10,000 were classified as anomalies to seepage depending on individual company interpretations. With the advent of 3D surveys, it is easier to locate seepage-related seabed features using seafloor amplitude extractions related to migration conduits, such as faults associated with shallow salt features or deep-seated faults. As seepage sites are now better defined and as we have an extensive geochemical database, the old scale for background versus anomaly versus seepage has changed. By correlating true seepage to reservoired oil, most “anomalies” are not related to seepage or to the reservoired oils, and therefore, are not related to the subsurface petroleum system. The biomarker signatures can be used to define source origins, and when merged with regional understanding of source rocks in the greater Gulf of Mexico basin, a deep water source model can be derived. A 2D TemisPack model confirms the seepage results based on a deep water source rock model placing the primary source centered on the Tithonian and possible secondary source rocks at the Mid-Cretaceous Unconformity (MCU) and Oxfordian levels. Based on oil-to-seep correlations, we can demonstrate: That most piston cores <30,000 fluorescence represent background, 30,000–50,000 are low confidence anomalies, 50,000–100,000 are high confidence anomalies, and >100,000 are truthable seepage. Biomarker signatures of most piston core extracts having <50,000 fluorescence do not correlate to the reservoired oils; however, the number of cores that may correlate to seepage varies regionally. Fewer piston core extracts correlate to seepage in the eastern and central Gulf of Mexico, whereas more extracts correlate to seepage in the western Gulf of Mexico in the 30,000–50,000 fluorescence range. The effects of recent organic matter contamination also differ in other basins, but its effects remain the same, just the thresholds for truthing the extracts are different. Geographical differences exist. A pervasive background biomarker signature is present across the Gulf of Mexico, related to either river discharge sediments containing extractable oil and/or organic matter, or possible sediment dewatering carrying an oil-like signature, unrelated to the subsurface petroleum system. There is a distinct pattern related to the Mississippi Fan. The “background signatures” appear to contain real oil, but do not correlate to the active true seepage. Using a rigorous approach when interpreting the detailed geochemical data from the piston cores, the “clean” seepage shows a regional trend that can be used to infer source rock type across the deep water Gulf of Mexico. In areas where clastic sourcing is prominent, lower sulfur oils are predicted, whereas in areas dominated by carbonate signatures, higher sulfur oils will be present.