Exploiting a very-high-resolution Archean surface record

巴伯顿绿岩带（南非）的穆迪群可能保存了一个罕见的，非常高分辨率的冲积，河流，海岸线，潮汐和浅海相的太古代表面记录，因此可能提供前所未有的洞察太古代环境动力学。穆迪岩群底部火山岩的沉积年龄与薄凝灰岩与河流砂岩互层的沉积年龄重叠，在地层上高出约2.5 km（3227 +/-4 Ma vs. 3231+/-6 Ma），与持续高但平衡的沉降和沉积速率的几个地质迹象一致。高精度测年的初步结果也表明，+/-1 Ma的分辨率，需要可靠地区分可定年的太古代事件可能只有几个Ma的差距，是可行的。因此，推断穆迪群的总体平均时间分辨率可能在~1毫米/年的量级上，并保存了非常短期的太古代环境动力学档案，如气候、季节和潮汐变化。本提案中要求的资金将主要用于实地工作，以测试这一假设的各个方面。目标包括：（1）可重新测定的地点并扩大其数量;（2）确定和取样古土壤地点，包括穆迪熔岩顶部的一个突出的古土壤;（3）调查厚的交错层和生物层沉积物中的潮汐节律。

The Moodies Group of the Barberton Greenstone Belt (South Africa) likely preserves a rare, very-high-resolution Archaean surface record in alluvial, fluvial, shoreline, tidal and shallowmarine facies and may therefore offer unprecedented insights in Archean environmental dynamics. Depositional ages from volcanic strata at the base of the Moodies Group overlap those of thin tuffs interbedded with fluvial sandstones stratigraphically ~2.5 km higher (3227 +/-4 Ma vs. 3231+/-6 Ma), consistent with several geologic indications of continuously high but balanced subsidence and sedimentation rates. Preliminary results from highprecision age dating also show that +/- 1Ma resolution, required to reliably distinguish between datable Archean events possibly only a few Ma apart, is feasible. The inferred overall mean temporal resolution in the Moodies Group may thus be on the order of ~1 mm/year and preserve an archive of very short-term Archean environmental dynamics such asclimatic, seasonal, and tidal changes.Funds requested in this proposal will be used primarily for field work to test various aspects of this hypothesis. Objectives include (1) resampling datable locations and expanding their number; (2) identifying and sampling paleosol locations, including a prominent paleosol at the top of the Moodies Lava, and (3) investigating tidal rhythmicity in thick cross-bedded and biolaminated deposits.