Rapid Crustal Uplift at Birch Bay, Washington

Geomorphology and coastal marsh stratigraphy suggest late Holocene uplift of the shoreline at Birch Bay, located northwest of Bellingham, Washington, during an earthquake on a shallow fault. LiDAR images show a raised, late Holocene shoreline along Birch Bay, with ~1 m of elevation difference between the modern shoreline and the inferred paleoshoreline. Commercial seismic reflection images reveal an anticline in Tertiary and possibly Quaternary deposits underlying Birch Bay. NW-trending magnetic anomalies are likely associated with the Birch Bay anticline and other nearby structures. Taken together, the geophysical data and lidar images suggest uplift of young deposits along a NW-trending blind reverse fault. Stratigraphy from Terrell Creek marsh, located just south of Birch Bay, shows freshwater peat buried by lower intertidal muds, indicating local submergence ~1300 yr BP. Stratigraphy of a 70-cm sediment core from Birch Bay marsh, sitting astride the anticline imaged with seismic reflection data, shows mud buried by detrital peat. One radiocarbon age from the core places the abrupt change from mud to peat prior to 1520-1700 yr BP. We divide fossil diatom assemblages straddling the mud-peat contact at Birch Bay into three zones. The oldest zone consists primarily of intertidal and marine diatoms, dominated by Paralia sulcata, Scoleoneis tumida, Grammataphora oceanica, and Gyrosigma balticum. An intermediate zone, beginning at the sharp contact between mud and overlying peat, consists of a mixture of brackish marsh and freshwater species, dominated by Diploneis interrupta, with lesser amounts of Aulacoseira sp., Pinnularia viridis, Eunotia pectinalis, and Paralia sulcata. A third and youngest zone lies in the upper half of the peat and is dominated by poorly preserved freshwater diatoms, mostly Aulacoseira cf. crassapuntata, Pinnularia viridis, P. maior, Eunotia pectinalis, and E. praerupta. Paleoecological inferences, based on distributions of modern diatoms, suggest three intervals of emergence separated by short periods of submergence. Fossil diatoms from 7 cm below the mud-peat contact show that the site began shoaling prior to the change from mud to peat deposition, possibly as a response to gradual uplift. Subsequent changes in the diatom composition within the peat may represent small submergence events, followed by gradual shoaling. The inferred shoaling suggests late Holocene uplift of the Birch Bay marsh over decades to centuries in a series of earthquakes rather than during a single earthquake. At least 2-3 meters of uplift is possible, given that sea levels were lower in the late Holocene when the uplift occurred. Our favored interpretation is that the uplift resulted from gradual release of accumulated strain on the anticline culminating in abrupt uplift during an earthquake. We reconcile stratigraphic changes at Terrell Creek and Birch Bay by having most of the uplift at Birch Bay occur in the last of the three uplift intervals (sometime after 1520 yr BP) at about the same time as submergence at Terrell Creek to the south.