Flysch deposition and preservation of coherent bedding in an accretionary complex: Detrital zircon ages from the Upper Cretaceous Valdez Group, Chugach terrane, Alaska

Abstract

The Upper Cretaceous Valdez Group represents the fl ysch facies of the Mesozoic Chugach terrane accretionary complex in southern Alaska. The Valdez Group is dominated by litharenite sandstone and argillite deposited as coherent beds, unlike the older McHugh Complex mélange and massive sandstones. Detrital zircons from fi ve sandstones sampled along an ~55 km transect through the Valdez Group were dated using U-Pb laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). The youngest populations from the two oldest samples, located along strike from each other, were 82-81 Ma. Three samples across strike and outboard of the others are separated by ~50 km, but each has a youngest population dated at ca. 68 Ma. All of these samples have major grain population ages that suggest erosion from the Coast Mountains Batholith, consistent with petrography and grain modes suggesting an arc source. No apparent age gap exists between the youngest McHugh Complex samples and the oldest Valdez Group samples, suggesting continuous deposition despite the different depositional and tectonic style. We propose a model in which the onset of coherently bedded fl ysch marks the transition from deposition in the trench or trench slope to deposition on the oceanic plate beyond the trench after it was fi lled at ca. 84 Ma, i.e., the time of the youngest mélange sedimentation. Preservation of coherent bedding resulted as large coherent blocks of Valdez Group rocks were imbricated into the subduction complex during continued subduction in Paleogene time.For permission to copy, contact editing@geosociety.org |

Evolving heavy mineral assemblages reveal changing exhumation and trench tectonics in the Mesozoic Chugach accretionary complex, south-central Alaska

“…While we concur with the interpretation of Kochelek et al (2011) that the Valdez Group represents sediment deposited beyond the trench that was subsequently imbricated and accreted as convergence continued, we also note that the switch to sedimentation of this style occurred shortly after the start of rapid sub sidence in the forearc basins of the Matanuska Valley-Talkeetna Mountains noted by Trop (2008). Capture of sediment in these depocenters would have reduced the sediment fl ux to the deepwater margin after the start of the Campanian (ca.…”

“…The moderate degree of deformation of the Valdez Group compared with the oldest parts of the accretionary complex precludes formation along the subduction channel for much of the complex. Only sparse fossil data exist to constrain the maximum depositional age of the Valdez Group (Plafker and Berg, 1994), but recent zircon dating now indicates a maximum age of 85 Ma, continuing until at least 68 Ma (Kochelek et al, 2011).…”

“…This gap in zircon ages suggests an Early Cretaceous subduction erosion event probably driven by a ridge subduction event (Amato and Pavlis, 2010). Kochelek et al (2011) performed U-Pb dating of detrital zircons from the Valdez Group and demonstrated that there is no signifi cant time gap between the sedimentation of the Valdez Group and the adjacent Graywacke-Conglomerate assemblage. They interpreted the onset of accretion of the Valdez Group as representing the fi lling of the paleotrench and sedimentation on the abyssal seafl oor.…”

“…9). Here, we plot data from the McHugh Complex Graywacke-Conglomerate from Worthman and Amato (2010) and from the Valdez Group from Kochelek et al (2011). Most of the McHugh sandstone shows slightly more lithic material than the Valdez sandstone and overlaps between the "transitional arc" and "undissected arc" fi elds of Dickinson et al (1983).…”

“…The bulk of that sediment was coming from magmatic arc sources, which zircon dating attributes largely to the Coast Mountains Batholith (Amato and Pavlis, 2010;Kochelek et al, 2011). The granite boulders seen in the Beluga Point conglomerate have generally younger ages of 180-200 Ma (Bradley et al, 2009) and suggest erosion from either the Talkeetna Arc or similar arc terrane.…”