ABSTRACT

Carbonate mud mounds in the Mississippian Lake Valley Formation, New Mexico, can be classified according to their external morphology, internal growth phases, and paleogeographic location along the Lake Valley ramp. Most of the Lake Valley mounds grew during a complete third-order accommodation cycle (Alamogordo-Nunn-Tierra Blanca interval; 3-6 Myr duration), which directly influenced mound biota, facies types, and the physical interactions between the mounds and sediment gravity flows that transported skeletal sediment around and over the mounds during the regressive phase of the cycle.

Lenticular mounds are found in updip parts of the preserved Lake Valley ramp, which largely consists of outer ramp facies. Most lenticular mounds grew exclusively during deposition of the transgressive Alamogordo Member, when outer parts of the ramp were somewhat sediment starved. Coarse-grained carbonate gravity-flow deposits (Nunn and Tierra Blanca members) typically onlap lenticular mounds and only locally interfinger with mound-core facies. These relationships indicate lenticular mounds were buried by carbonate gravity-flow deposits, which accumulated much faster than mound cores could aggrade. The sediment gravity flows constructed shingled lobes, sand sheets, and oblique clinoforms and were erosive at times, which also probably helped to shape lenticular mounds.

Transitional mounds are typically located downdip from the lenticular mounds and had an initial lenticular growth phase, which was followed by aggradational growth of mound cores. Aggradational mound cores were locally modified by submarine erosion or failure, which created sharp stratal truncation surfaces that were later onlapped by gravity-flow deposits. After the aggradational phase and stratal onlap, mounds with remnant seafloor relief served as substrates for continued growth during deposition of the Tierra Blanca Member. This final growth phase in transitional mounds records minor aggradation and significant, typically asymmetric, lateral accretion, which suggests that: (1) the mounds had grown to some accommodation limit (possibly storm wave base), and (2) the complex dispersal and accumulation patterns for the gravity flows also influenced how mound cores accreted laterally. For example, the upcurrent sides of some transitional mounds were buried preferentially by onlapping Tierra Blanca gravity-flow deposits, whereas their downcurrent sides locally accreted for significantly greater distances and with very different stratal geometries.

Hemispherical mounds are located farthest downdip on the Lake Valley ramp. They are the largest mounds and contain all three (lenticular, aggradational, and accretionary) growth phases. Later growth phases of hemispherical mounds have steeply dipping mound core-to-flank relationships with abrupt facies transitions. Because of their distal position along the Lake Valley ramp and greater distance from the progradational sand bodies, the hemispherical mounds were the dominant sources for coarse-grained carbonate sediment to adjacent non-mound environments during Nunn and Tierra Blanca time. Thus, the distance of lateral accretion and overall morphology of hemispherical mounds were not affected greatly by allochthonous sediment from updip parts of the ramp.