56-6 Decoding Mixed Tidal Cycles and Sedimentary Structure Dynamics in the Neoproterozoic Siliciclastic Aaron Formation, North Carolina, USA

Abstract:

Decoding Mixed Tidal Cycles and Sedimentary Structure Dynamics in the Neoproterozoic Siliciclastic Aaron Formation, North Carolina, USA

A stratigraphic section of the Neoproterozoic (Ediacaran; 580-575 Ma) Aaron Formation, located along the South Fork Little River (North Carolina, USA) was studied. The rock succession displays sedimentary structure (microbial mats) alternating with background deposits. In the millimeter- to centimeter-scale stratigraphy, a tidal signal was detected. Petrographic analyses by light microscope and electron probe micro-analysis were conducted on thin sections. These derived from subsamples taken along an 8.75-cm transect crossing a rock sample taken from an outcrop. The analyses reveal alternating Si-rich and Fe-rich mm-cm-scale laminae that are relatable to a grayscale scan taken along the slab. A 4.4-cm-long section of the scan exhibits two co-occurring lamination frequencies interpreted as semidiurnal (M2) and diurnal (K1, O1) tidal constituents. Tuning the scan to the interpreted M2 constituent yields a 46-day-long depositional time series and reveals a weak ~14-day spring-neap modulation. Microbial-mat horizons align closely to neap-phase minima, indicating episodic neap-tide colonization. Comparison of the M2-tuned Aaron Formation grayscale time series with a modern mixed-tide record from Port Angeles, Washington (USA) shows close agreement between their power spectra, with evidence in both for M2, K1 and O1 tidal constituents. The M2-tuned Aaron grayscale time series frequencies are K1=0.996 cycles/day and O1 =0.9109 cycles/day, compared to the modern Port Angeles values of K1=1.002 cycles/day and O1 = 0.93102 cycles/day. The M2:K1 ratio of 1.95:1 of the Ediacaran Aaron record is close to the 1.93:1 ratio of the modern Port Angeles record. The frequency difference K1-O1 yields the fortnightly cycle frequency; the Port Angeles record indicates the expected K1-O1=1/(14.088 days), but the Aaron Formation record indicates K1-O1=1/(11.75 days), suggesting that the M2-tuning procedure was imprecise, or that some of the tidal laminations are missing (i.e., not recorded or measured), or both. Despite this shortfall, the results provide strong evidence for the preservation of a mixed-tide regime in the Ediacaran Aaron Formation locality. Microbial stabilization of the deposits leads to the retention of a well-resolved tidalite.

Keywords: Tidal rhythmites, Sedimentary Structure, Stratigraphy, Semidiurnal-diurnal tides, Microbial mats, Spectral analysis, Siliciclastic tidal flats, Neoproterozoic.

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