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Reports: ND852790-ND8: Hyperpycnal Subaqueous Fans of the Northern Santa Barbara Channel, Central California

Alexander R. Simms, PhD, University of California, Santa Barabara

The overall purpose of this grant is to study the morphology and deposits of recently imaged seafloor fans on the inner shelf of the Santa Barbara Channel.   Our central hypothesis is that they are created by hyperpycnal flows.  We have 3 major aims: 1.) map the distribution of one of these features using high-resolution seismic profiles, 2.) obtain grab samples, underwater camera images, and shallow cores to study the sedimentary characteristics of the deposits, and 3.) compare the petrology, grain size, and shape of the sedimentary deposits to sediments from the possible local sources.  Over the past 18 months we have collected ~50 km of seismic data, 8 cores, and 9 grab samples (Fig. 1).  All cores have been cut and described and 4 cores have been x-rayed.  In addition, we have conducted grain size on 107 samples from the cores.  In addition we have sent 5 samples to an outside lab for radiocarbon analysis.   The cores collected from the Refugio and Tajiguas Creek fans in the Santa Barbara Channel contain four facies (Fig. 2). Due to an overall lack of sedimentary structures, facies are distinguished primarily by variations in organic material and shell fragments.  Facies 1 contains very few to no shell fragments, occasional intact shells, and a moderate amount of organic fragments. Facies 1 is present near the top of all cores and comprises all off-fan samples. Facies 2 contains a moderate amount of both shell fragments and organic material. Facies 2 also contains occasional articulated bivalves and intact shells. Facies 3 is composed of abundant shell fragments, abundant plant fragments, and occasional granule-sized material. The final facies found in these cores is facies 4, which is a fine-grained sand layer with some bioturbation. Facies 1-3 contain clayey, very fine-grained sand. In contrast to facies 1-3, facies 4 is a clean sand and exhibits sharp upper and lower boundaries.     Seismic profiles through the fans reveal at least three seismic units.  The basal seismic unit consists of steeply dipping reflections unconformably overlain by mounded and/or chaotic facies (Fig. 3).  The overlying mounded and/or chaotic facies appears to have another through-going surface separating units of very similar seismic character.  Our preliminary interpretations are that the lower unit composed of steeply dipping units represents the underlying Miocene bedrock and the overlying mounded/chaotic facies represents Holocene sandy deposits found within our cores.  The unconformity separating the two is thought to represent the amalgamated sequence boundary/transgressive surface.  The through-going surface within the upper two units likely represents either a lobe boundary within the fan deposits or the basal surface separating ambient Holocene shelf sedimentation from the deposits of the fan.    Thus far, we are leaning towards an alternative interpretation for the fans than our initial hypothesis.  Our current working model is that the fans are composed largely of debris flow materials that initially left the mouth of the river as hyperpycnal flows (likely represented by facies 4) and transitioned to debris flows either through flow separation and or remobilization of the deposits initially deposited in the shoreface.  We have also found a strong role for shelf gradient in the formation of the seafloor fans as areas with gentler slopes along the channel appear to be devoid of these seafloor fans.  The steeper slopes that appear to be necessary for the formation of the fans appear to be related to the extent of the shoreface.   Future work will include comparing the petrology of the sands from the fans to those of the beach and creeks as well as continued grain-size analysis and selection of materials for 14C dating.  We will also quantify the slopes in which the fans are formed and those in which the fans are absent.  We also plan to examine the slopes of the toes of the fans in order to determine if they are consistent with those expected by debris flow processes.   Our results were presented at the American Geophysical Union Fall meeting in December of 2013 and will also be presented again this fall at the next annual meeting of the American Geophysical Union.  Both presentations will/have been first authored by Elisabeth Steel, the PhD student supported by this grant.

Figures

Figure 1.  Location of all cores and grab samples collected on the subaqueous fans of the northern Santa Barbara Channel.

Figure 2.  Facies found within the cores collected on the subaqueous fans of the northern Santa Barbara Channel.

Figure 3.  Examples of seismic facies found in seismic surveys across the subaqueous fans of the northern Santa Barbara Channel.